All depends on the motor! Usually for 17.5 and 13.5t boosted classes I basically turn everything up to 100%. I.e. Max timing, full turbo with a 3 or 6 degree slop depending on traction level with the boost generally starting at 3-5k RPM. This is on a speed passion reventon pro 1.1 (v21 stock1 s/w). I leave the timing stock on the can which is usually about 10 degrees or so. You need to be very careful with gearing and watch the temp on the motor. You'll need a good 40mm fan on the motor as well. ORCA and Speed Passion motors take the boost quite well I've found so far.
Motors lower wind than 13.5 will need the max timing turning down to suit. Key thing is watch the motor temp! Remember the timing boost set in the esc is additional to the motor can timing!
Modified motors around 4.5t require far less boost and turbo than 'stock' high turn motors so don't ever use the same settings for a 13.5 stock as you would with a 4.5t modified!