Using the manual speed control POT on the spindle controller, this is the max speed I get from the 400w spindle from SMW3D. Disclaimer: The tachometer is from Harbor Freight so…
If you are using tinyg with pwm, you will need to play around with the pwm duty cycle variables $p1cpl and $p1cph to get closer to 12000. A higher value of $pqcph e.g 0.5 will get you there.
I use the same spindle and measured approx 11700 with an rpm app that works with audio not sure how accurate it is either but so far I just set it to max rpm and adjust everything else
Give this a look. https://dl.dropboxusercontent.com/u/50261731/Spindle%20Test%201.doc
It might be dependent on the controller you have for the spindle.
Ha- I was about to suggest @cmcgrath5035 's awesome guide.