if you have confirmed the dampener slit on on true TDC for #1 or #6 you could back up the crank a few degrees then come forward till the slit is on TDC & see where the reluctor tooth clocking is at in relation to the magnet then hand bump the crank forward till the next tooth is in the same relative clocking to the magnet (dont go backwards cuz that introduces the chain/intergear slop. A person could also get a cyl as close to TDC as you can with a rod in the cyl then make a paint mark on the dampener at that point on zero on the timing tab then back up the dot to 15BTDC & screw in the TDC tool to firmly contact the piston then go CCW & see if it contacts at 15ATDC & if not exact then split the difference in the numbers to find exact TDC. Might need to start at 10BTDC so you have room to work (been a long time since I done it). EDIT if the cam is symmetrical you could as you mentioned get "rockover", the point where the lifters on overlap are both moving and are at the same height & that cyls' piston and its companion cyl should be at TDC but there is still the chain slop issue as you are going back & forth a bit to get the heights equal. Assuming you want dead on as opposed to very close. MORE EDIT (1) confirm TDC dampener slit is true. (2) cut a strip of white paper 5.694" with your calipers (3) use it as a ruler on the dampener and a sharpie marker to get to/mark each of the next cyls' TDC, to #8 then #4 etc. (4) Only need (3) ink marks along with the TDC slit.