Trial and error, guided by test data. Some degree of artistry does help!
The OEM axle length too often bears no accurate relation whatsoever to what optimum Reboxx (or other brand) replacement wheel sets might work best. The actual shape of the molded plastic bearing holes vary all over the map, the variety of shapes ranging from near-cone-like, to shaped like rounded-over cones like a dull lead pencil point or the classic "Tee Pee' sawdust burners or logging fame, to bearing cavities actually resembling near-domes. Virtually none are true cones. As a result the sharp ends of the pointed axles will optimally ride anywhere along the varying sloping sides of the bearing surfaces. As a result, the best rolling characteristics occasionally result with relatively short axles riding on the widest parts of the bearing cone. in turn effectively lowering the car height, and allowing excessive axle end play ("the drunken sailor gait"). On the other hand, when axles are fitted optimally to fill the bearing holes to their very apices at both ends to reduce end-play, rollability can suffer because the pointed end of the axle is having to ride on an unfriendly bearing surface.
Even then, axles of the same nominal length but of differing manufacture, diameter, or material can result in far differing test and real time data.
Ditto with trucks that come out of differing mold machines, and are made of differing materials.
1) The value of the reaming tool is overestimated, but it is not valueless. I have rarely found that its casual use actually improves rollability in most instances, although the effective "cleaning out the hole" can be satisfying in and of itself. Because it is a machine tool, I *have* found it useful to use in a drill press to deepen and shape bearing holes in brass and metal trucks so that they actually accommodate any sort of replacement wheelsets.
2) Accurail "Bettendorf" trucks: 2x increased rollability to the Superior range with minimal end play (with minimal end play) can be reliably attained with 1.025" replacement Reboxx wheelsets. An occasional truck might need 1.020" wheel sets, or even a mix or two different axle lengths in the same truck to attain the same results (they are produced by four different molding machines, if I interpret the numbers on the bolsters correctly)- but these occurances are rare.
3) Kadee "Bettendorf" (metal): 2x increased rollability to the Very Good range can be reliably attained with 1.020" replacement Reboxx wheel sets.
4) Central Valley "AAR" (metal): 2-1/2 x increased rollability to the Acceptable/Good range can be reliably attained with 1.015" or 1.020" Reboxx replacement wheel sets (reflecting CV's individual truck variation).
5) NWSL replacement wheel sets: Sadly, these beautiful replacement wheels sets do not ordinarily roll any better than the OEM wheel sets, and sometimes not as good. IMHO, this has to do with the relatively fat axle ends, and the fixed 1.015" axle lengths. I have used them on some cars that are going to be relatively unmoved layout show pieces, but more commonly I have purchased the fine 0.088" NWSL wheels by themselves (notably the unusual 28", 30", 40" sizes, etc.) to then press on to selected Reboxx axles of optimal length.
6) Tahoe trucks: They are designed and molded to specifically to fit IM replacement wheel sets, which for years have measured out to 1.012", although nominally 1.015" (identical to the Reboxx axles of the same length). The trucks reliably operate into the Superior level with these wheel sets. However, IM (not Reboxx) changed their wheel set manufacturer (without also changing the packaging or labeling) and the their new wheel set axle lengths are now 1.007". I solve this irritating matter by purchasing the truck frames by themselves and installing Reboxx 1.015" wheel sets.
6) Reboxx tells you, and I too can confirm that their nominal axle lengths are commonly =/- .003 or .004". Although I cannot say that I have measured each and every Reboxx axle -or even come close- I have replaced -or installed new- about 1200 Reboxx wheel sets without coming across any oversize axles, except as I might have come across wheel sets that I misfiled during some previous testing.
7) Because of the reduced gathering ranges of the scale couplers, -Accurail, Kadee, Sergent- excessive wheel set side play can and will play havoc with reliable coupling.
Denny S. Anspach MD