I use 60' or 65', eyelet to eyelet.
BUT: I build heavy and I compensate by flying fast, and then I compensate for
that with long lines.
I'd get the lap times up to 5.2 seconds or so, then see what you think -- if the joy you get from actually being able to keep up with the plane in maneuvers in exceeded by the dismay of having it fall out of the sky in the corners, then you may want longer lines and a bit more juice in the engine.
I strongly recommend that you measure your lap times and at least try things out at various speeds from a lap time of 5 seconds and getting progressively slower until you just can't stand it any more. Speed masks a lot of trim problems, so you may find that it's a dog at 5-second lap times. If so, work on trimming the airplane at that speed before you go slower. Then consider that changing speed changes trim requirements, so as you go slower consider that you may need to at least tweak the trim.
I'm flying a Brodak 40 (10x6 prop) on a 526sq Ringmaster right around 5 seconds on a good 2-4 break with 60' lines. If I lean it out, times go down to 4.5. If I go richer, I can get 5.2. I'm a firm believer that the engine mixture and prop load should be used to set your lap times, not line length.
A quick bit of math: Changing line length by 1 foot only changes the lap time by 0.08 seconds (assuming we start at 60' and 55 mph). So adding 5 feet (60 going to 65 ft) for a fixed speed only gets you 0.4 seconds. I think it's much simpler to adjust the fuel/mix/prop to make that adjustment.
Well, yes, but you're adjusting two different things. I agree that you shouldn't just lengthen the lines just because you can't get the engine tamed, but that doesn't mean that you shouldn't make sure you're flying on lines that are right for you.