My approach was to take a benchmark locomotive, run it at top speed over a timed course to calculate its speed in scale mph, figure out where I wanted top speed to be as a percentage of that (i.e.100 mph actual versus 50 mph desired + 50%) then adjust cvs 5 and 6 to that percentage. Test run to see how it performs, then make any adjustments from there. Once that was done, I'd do the same to the rest of the fleet, and tweak them so they ran at exactly the same speed as the benchmark loco. I run diesels mostly, so speed matching is necessary to run consists. For steam, unless you intend to doublehead them it's not as critical that they perform exactly alike.