I would start with 60 or 61 feet eyelet to eyelet. You can easily shorten them from there if need be. They haven't invented a line stretcher yet! If you haven't flown a model with an LA.25 yet, it will surprise you with what it can pull. I don't quite go as far as the claim "same as a Fox .35" because they runs so differently and make a different kind of power. I have a SIG Twister that I fly now and then with an LA.25 on it. The Shoestring is smaller, a bit lighter and I think a cleaner design, so you don't want to make the lines shorter than you have to. Before I re-engined my P-Force, I flew it with LA and FP .25's and I was at least 60 feet eyelet to eyelet. Use a APC 10-4 prop, about 10,500 take off RPM, 10 or 15% nitro fuel, and have at least a 4 ounce tank on it to do the pattern in cooler weather. That is pretty much the settings for anything I have flown with the .25 engines, including a Top Flite Flight Streak ARF, which is about the same size and weight as the Shoestring.
Type at you later,
Dan McEntee