You might also try Weiss Cannon UltraCable as a cross with gut or multifilament mains. It's also a square section poly, but of a different composition than typical co-polys. They recommend stringing 1.5kg lower than with typical co-polys, and I've gotten good feeling results stringing the crosses 8 lbs lower than the mains, at around 56 / 48. It sounds like a lot, but makes sense (±10% lower for a poly as compared to a multi or gut, so 5 lbs lower, plus the 3 lb lower recommended by Weiss Cannon, for a total of 8 lbs). If you want the mains to feel 2-3 lbs tighter than the crosses, then go 10-11 lbs lower. I've only done this on relatively tight string patterns, namely, the Donnay Pro One Penta 18x20, Yonex VCore Pro HD, and HEAD Prestige Pro 16x19. It could be worth a try on a more open string pattern, but I think I'd want to go tighter.