Here's a real, empirically derived utilization curve:
Utilization is a continuing source of confusion. There is the utilization in the boil, which depends strongly (and asymptotically) on time and moderately on inverse gravity. (There is a temperature component as well, but unless you are using a pressure cooker or live way up in the mountains, it can be ignored.) And then there is the maximum bittering achievable, which depends weakly on specific gravity. Higher gravity means a higher maximum bittering, but it takes much longer. Lower gravity means a lower max., but you get there sooner.