With the engine at full throttle, and max rpm reached (engine installed in plane), take note of the pump PW shown on the terminal. In the run menu, set the pump limit the the value noted at max RPM + 10-15% (ie; if max rpm was 500, set limit at 550-575). Accel Delay can now be set: advance throttle from idle to full quickly, each time lowering the delay value. When you notice a change in sound during acceleration (sounds a bit throaty, or you may see some flame), stop at that setting and add back two points. Cooler, dense air will allow for faster acceleration; warm thin air will require a slower setting.

Just an FYI - Pump PW reading on "Brushless" motors like a Merlin is actually pump RPM, as opposed to a voltage on brushed pumps.