Your method seems sound. Trust your numbers and start from there. Put the bolt in, measure again with your dowel. Subtract the measurement with the bullet from the one to the bolt face and you have a "max"coal.

A better measurement would be to measure the ogive with a bullet comparator, not calipers. Tips will be inconsistent, even with the plastic tips. Measure bullet base-ogive. Measure bullet base to tip. Do more math and you can get a pretty good idea of the o-give to tip length. the lengths may be closer than they appear; if they are not, you may need to alter your approach.

I think you have a good starting point. I would take your "max" coal from above and load a dummy round, no primer, no powder, to that length. get the bullet good and black, with a sharpie or you can smoke it with a candle, and push it up into the chamber with your finger. If it sticks, you are probably in the lands. Use a rod to push it out and look for where the black is gone, these marks will be the lands. From there you can progress several ways. I seat the bullet .005 deeper (measureing cartridge base to o-give; cb-ogive) and repeat the sticking it in the chamber. When it no longer sticks, you are free of the lands. If the possibility of error of .005 is good enough for you, then measure cb-ogive again call it zero. Anything shorter than that will be off the lands, anything longer will be jammed. Use the same bullet comparator, they are not always the same.

if I am not making sense, and you would like a better explanation, let us know and somebody can help fill in the blanks.