JFP - Success! (I think). I used your method this afternoon and got the RMS in very, very accurately with a minimum of fuss.
I reused my old flywheel bolts as part of the "JFP Tooling". Four bolts fit very nicely inside the PVC coupling, grabbing it by the internal ridge intended for the PVC pipe to bed against. Turns out the thread pitch for flywheel bolts is is 1 mm per rotation, making it incredibly easy to get the RMS at whatever depth you need. After sliding the RMS off its mounting ring and onto the crankshaft end, I rested the coupling on the RMS and started each bolt with a half a turn to get the theads engaged. I counted turns as I spun the bolts in equally until the RMS was flush with the block. I took the whole thing off and checked the depth from the crankshaft face: 9.9mm in my case. I put the arrangement back on and turned the bolts three more turns each (1/2 a turn at time to keep things even). Popped the JFP Tooling off, checked the depth, and had a consistent 13 plus or minus .1 mm depth. Wow. I counted turns coming out at the intermediate step, so that I could quickly get back to the same depth for the final push.
Beautiful stuff.