Mark,Both boats are not in the same lane. They are 10ft apart which is more than a lane but very close (35 & 45 ft radius turns, the straights are the same no matter what lane).
I use the 5 mph rule because I think I can get a good visual on that and sometimes I know the speed that the guy I am racing is actually running. So if I am behind somebody that I am not more than 5mph faster then I will just hang back and hope he makes a mistake because I will never pass him otherwise.
Ultimately John is right because it is the fastest lap time. You have to judge how much water you gain after a lap to be able to make your decision.
This is why even 100mph boat means nothing if it dies off in the turn.
My gas cat runs 52mph. Whoooo Hooooo! Right? Well, I have to tell you that nobody has ever passed it if I am in lane one. Well, Earl Eber did it with a Seaducer. In the turns I am below half throttle. So this tells me that these 60+mph cats either are not running that fast or they do not turn for snot. BUT! If I am not in front in lane one then I loook like I am sitting still.
I think John's last set of record boats were actually slower in the straights but obviously faster in the turns since he took a second of the oval times. Slower can be faster.