Bottom-ish of the range.
Just remember: the whole point of this setup is to get lots of controllable power (ie power + spin). If you find you're hitting too flat, you may need to make a small adjustment or two so that you're generating more top.
EDIT: Having thought about this further, I wanted to add a caveat since I don't want to give you bad advice: if you have a late 80s style OS racket to go with late 80s style swings, what are you looking for by switching to a more modern style string setup? If you purely just want to tame the power of full bed gut, why not just increase their tension?
This string setup is very complementary to more modern swings: open stance eastern to semi-western forehands with Federer- or Djoker-like swing paths. So, for example, if you put one of these hybrids in McEnroe's racket, I'm not entirely sure that he'd benefit that much from it. But then again, Sampras seems to love full-bed poly in one of his new Babolats. So go figure.
I wish I could offer you more certainty, but you're caught between two extremes here: (1) at higher tensions the stringbed locks up and you don't get as much spin because spin is dependent on the mains deflecting and then snapping back into place on impact, and (2) there is such a thing as too low wherein if you don't generate enough top you'll be launching moon shots.
Of all the reviews I've read of gut-poly hybrids, I don't think anyone has reported using an OS racket before, so realistically you're probably going to be something of a guinea pig here.
The risk of going too high is that while you may think it's ok you won't be getting the full benefits of spinniness. So my advice would be to go low, bottom-ish of your range, and then if it's still too powerful try putting in some string savers. That said, if you're not interested in potentially having to adjust your swings to generate more spin, then you should probably just stick to full bed gut and string it higher.
6 x Head MGPMs, 3 x Head i.PMs, Gamma X-ELS
Last edited by Smasher08; 09-18-2011 at 08:20 PM.