I've got the lifts in what I think may be the perfect spot now. Without upholstery they're powerful enough to lift open the top by themselves. However, once I put one of my sofa cushions on it, it stays down. The actual upholstery on the final product will be pretty much the same materials, only about 50% more of them. That should allow for a fairly light lifting action without any accidental lift-offs.

Needless to say, there was quite a bit of trial and error involved. Oh, and "by feel," the difference between 1 and 2 lifts (in use) is dramatic.