A few of you are dimly aware of my continuing saga with my 1918 Hobart M. Cable upright - replacing parts, getting it tuned, fixing it up, blah, blah.

I'm having a lot of fun with it - and now I'm deep into regulation. I regulated for lost motion which made the most incredible difference in how the instrument plays. Now I've started to regulate the let-off. The let off was WAY early; I assume because of all the lost motion I took out in regulating an 89 year old piano. Also, the back checks are WAY too close; rather than the 5/8" hammer check specified in Reblitz, its more like 1/8". A few keys are now on the verge of blocking due to the backchecks.

Sanity check please - am I proceeding normally for a piano this age, or am I doing something stupid?

Hammer travel, key dip and let off should be set before anything else. Then set the lost motion after you are sure none of the above will change. After the action plays evenly then set checking.
Use a wire bender to do the checks and if they are interfering with regulation because they are too close then just move them back till later.

Gene's right, you've got the action set-up sequence backward.

Some things to consider:

1. You may need to fiddle with the height of the keys. I am assuming you have replaced the original felt punchings on the key pins, but you may have a problem being certain of the original key height (size of punchings and thickness of backrail felt). If you are using the original punchings and backrail cloth, they are no doubt much thinner than they were 90 years ago and may need to be replaced for the action to work well again.

2. If you have filed the old hammers to regain proper shape, you will probablly have a significantly shorter hammer (strike point to shank) and longer hammer travel distance. Some techs compensate for this problem by shimming up the hammer rest rail. The best approach is to hang a new set of hammers, bored to the original dimensions (or as close as you can estimate). Smaller hammers will also change the hammer strike line which can have a significant effect on sound.

3. So long as you are having fun, sanity is not an issue.

Lost motion must be taken out before you can set most of the other measurements, particularly key height and key dip. But you may need to check it again later.

You cannot just prop the backrail forward to compensate for worn hammers. That has an adverse affect on the damper regulation. It is a good idea to replace the back rail felt, but hammers need to be replaced sooner on uprights than on grands.

Letoff is usually early because of wear. As I have said before, it is a snap to set it if you make a strip of magnetic tape and cardboard which totals 1/8" thick and stick it to the strings at the strike point. Set the letoff so the hammer just touches the strip.

Back check felt and catcher leather do wear out. If I am doing a complete job on an old piano, I replace them both. If they are in good condition, you can usually set the backchecks properly by bending them back towards you a bit, and then pressing them to just touch the catcher, and letting them spring back. It is quick to do evenly and leaves them reasonably close to the proper measurement.

Backrail felt has been replaced, front and center rails punchings ditto - they all were, err, disgusting. Keys have been leveled (and rebushed). Hammers received a light reshaping. Strike distance is at 7/8", which is on the outside edge of acceptable as I understand it. I'm setting let off to slightly better than 1/8" due to the presence of a felt practice mute. I'm using the "magnetic method" - it works great!

I dialed out lost motion first as that is what Reblitz tells me to do...

But the message I'm getting here is that the early let-off would be normal, considering the piano's age. What about the too close backchecks? I "think" I have the key height set approximately right - the slight yellowing of the fronts forms a line that matches up with the keyslip.

I certainly have no problems with bending the backchecks - I'm just looking for some confirmation before I "regulate myself into a corner"...

Lost motion doesn't have to be set to do key dip or key height in an upright. The hammer has nothing to do with the return of the key and the jack only contacts the hammer butt on the blow. The wippen and leading of the key returns the key.
Key dip has to be set properly before you take up the lost motion because the jack jams against the stop rail. Also its a good indication that you have too much key dip when the hammers are checking too close to the strings. Reduce the key travel and the checking will come back to where it should be and you won't have mangled the wires out of adjustment then have to reset them all.

That is wrong. You can take out the lost motion and see the keys rise on most upright pianos.

If there is one adjustment that does not change during regulation it is letoff.
How many things will affect lost motion? Key hight, changing back rail felt, key capstan, hammer rail position, changing hammer rail felt.
You have got to ask yourself how many times you want to adjust lost motion.
Has anyone had the experience of setting lost motion on an upright with no key leads? The new Stagerman's for example. Almost impossible to do it accurately. The whips are just not massive enough to return the key to proper height without the lead.

If you replaced the back rail cloth with the incorrect size, the key dip could be too deep, which will make the hammers check too close. Please check your key dip.

Will do.

Backrail cloth was "about" the same thickness as the chewed up crap that was there - slightly thicker if anything. But key dip a-checking I will go!

Before you get deep into regulation... - oops, too late.

Better late than never, here is my 2 cents worth, for the next time you try this:

Instead of regulating the whole piano, do a test regulation of three notes from start to finish. That way you will know if it will work out.

Select two adjacent naturals and the sharp they flank, in the low tenor. Proceed in this sequence (works for me)
set:
blow distance
lost motion
key height/level
let-off
dip
checks
spoons

Check for after-touch. How does it play? repeat 85 times. Done (rough in).

Go through it all again, to re-fine and check.

Keep having fun