Trouble is, if the case is sticking slightly in the chamber, the bullet may be pushing back into the case before the case starts to move.
Edit:
I see you were editing while I was replying.
It sounds like you have a good number to start from. Give it a try and watch for pressure signs.
Here's another method to try....
Chamber the dummy round as you did originally. Slide your cleaning rod, with a flat tip on it, into the barrel until it touches the bullet. Scratch it with a razor blade, held flat against the crown. Now eject the round, close the bolt and insert the rod until it touches the breachface. Mark it again with the razor. That should give you a good number too.
The discrepancies you're seeing are why I greatly prefer the Hornady OAL tool.
As far as the off-set from the rifling, nothing wrong with 0.020, it's just that there's really no "correct" number. What your gun "likes" will change depending on the bullet and powder, or it may not change. Most of my guns seem to care more about consistency that a specific number.