What I can give you is my personal experience. We have our job easier in Europe because we are allowed to run only 12 turn motors.
From various tests I saw that a good 12 has a max rpm of aprox 38-40k rpm.
When I test a motor an compare the curves I can see that the power curve has it's max further ahead on the rev range, so I need to allow the motor to rev a little more and that's done going down in pinion.
When I got my first Kr I saw that it was able to do 45k rpm and comparing curves with a Ti for instance the max power was almost 5000 rpm ahead the Ti power curve. The conclusion I took from that was that instead of using a high 5.0 ratio or a low 6 would melt the motor because at the same point of rpm it had much less power and torque than the Ti. And if a motor is too loaded has the bad habit of heating and melting.
I had to allow the motor to rev more. I used a pinion 2 teeth lower and while my Kr is still in my box, 4 new Kr's melted in that race coz the guys were using the exact same ratio that they used with the Ti. They had no idea of the differences when they installed the motor on their cars.
6-7K rpm more is a lot. Too much to use the same ratio. Only a dyno will tell you this, unless you're gifted
The exact same pointers I got with a D5 and now with a Pt. I'm not a dyno god and I only use it to get some ideas that I need to prove on the track. If they are sucessful more than once then I may be doing something right.
I've beaten my personal track record (on my usual track) with my Kr while all the others had a hard time making it run decently.
The dyno just pointed me in the right direction, but the track testing proved I made the right assumptions.