Anymore, the safe answer is to buy a couple of each size and length wheel set to have on hand and figure it out by trial and error. Sometimes even the "correct" wheel set doesn't feel right. It's good to have options on hand.
To be completely frank, I've more or less given-up on trying to replace wheelsets ad-hoc like this on Atlas cars. If I have Atlas cars I want to retrofit FVM wheelsets to (and there are a lot), I'll retrofit MTL trucks. I keep all the FVM axle lengths in my bin, but I've had frustratingly variable results with Atlas trucks rolling to my satisfaction, even when I've used the "correct" lengths, and then tried other lengths after calipering the originals.
That Atlas through the years has changed axle lengths as they have managed their supplier network doesn't help. It's a crapshoot to start with.
