Do you know the factory BTDC value for a stocker to start from. If mods done like compression increase, ports raised, etc and fuel octane and type and oil mix all make a difference. The way I adjust timing on my race engines is pick a start value (like stock) take a run and record speed and rpm, kick it up a small amount and run it and get results. Keep going until speed and rpm drop. To do that I make runs long enough like 1/2 mile so the engine gets heat soak which effects advance needed. Need to be careful with advance piston holes could result. 2 cycles like less advance at top speed than more, more generally increases lower end power and acceleration. How you use the boat and motor matters.