I always used a dial indicator and a 2mm timing pin to time my 80 390CR. I have a dial indicator with a sparkplug adapter on it that threads into the sparkplug hole. I would insert the indicator, rotate the flywheel watching the indicator to get to TDC, zero out the indicator, then back it off 1.8mm BTC and insert the timing pin.
If it didn't line up, I removed the flywheel and adjusted the motoplat, put the flywheel back on (no nut) and check it again.
Not sure what the timing is on the 250, but the 1980 390CR/OR was 1.8 BTC, this was the 2080 motor.
Steve :cool: