Set up the action on a flat, stable surface. This will never be the same flat as the keybed, so don't worry about small imperfections. As long as the surface is hard, more or less flat, and stable, these sampling and setup techniques will provide an on-bench replica of the in-piano bedding and strike so work will fit in the piano.
Fix the action position side-to-side and at the back, leaving the front open to slide the action in and out as needed. Small shop scraps and double-sided tape work well for this. Grandwork Tools now offers a kit with suitably-sized aluminum pieces and a roll of tape that firmly attaches the pieces but can readily be removed (not true of most double-sided tapes). A Custom Keybed has stops instead for fixing the front-to-back position and slipper tracks that hug the action for a fixed side-to-side position. The frontrail stops flip down for keyframe removal.
In both cases, shimming of the rails will reproduce the in-piano bedding but can only be guaranteed accurate for that location. And in both cases, the stops will need to spatially cooperate with other tools used in the regulation. At the sides, for instance, use longish, narrow, straight pieces whose inside surfaces hug the action in place and whose outside surfaces are hugged by the Regulating Rack and allow it to slide into the various front-to-back locations that will be needed.
A 60" x 30" sheet of MDF can be used for this setup, with the latest array retained and reused or at the ready for repositioning. The sheet can be hung on a wall, slid behind a bench, or stored behind a door or cabinet for small shop flexibility.
At the back, the disposition shown in this photo firmly keeps the front-to-back action position while leaving maximum spatial flexibility. And the stops can be carefully leap-frogged for same action location with different points of contact.
Having checked the backrail of the piano's keybed with a level stick, estimate the bench shimming of the backrail. Typically, the backrail will end up flat. You can tell by the shims in the photo above that the table has a swale. Although the topstack reinforces the keyframe's backrail, making it quite rigid, its overall length can sag, enough to cause setup problems. Touching up the shimming points later provides a backup option for front and balance rail shimming difficulties. The bedding sample Keysteps will arbitrate and whatever the tweaks - when the Keysteps say yes, the regulation can proceed.
A gentle clamping at the keyframe guide pins acts as the cheekblock holddown function. Particularly for a crowned frontrail fit, this may be necessary. Least will be best here for the same mechanical leverage reasons we bedded the cheekblocks to not overbear.
And one little piece more at each side completes the positioning setup: add a short stop in front of each Regulating Rack foot (once strike is in place) to be able to return the rack to strike position easily with no additional setup. Because the arc of hammer travel moves forward from rest position to strike (the hammer crown's elevation at rest equals that of the hammer center, at least for custom bored hammers), all regulating will happen at or behind this position.
Next week: Bedding Setup on the Bench