For all my IPAs, 100% of the bittering comes from a single, large addition either right at flameout (for my west-coast IPAs) or after cooling to 160 degrees (for my hazy IPAs, where I'm going for less bitterness). I suggest you try this.
I get the right amount of bitterness while maximizing the hop punch--which is the whole point of an IPA, yes? Which steeping temp to use depends on how much bitterness you're after. But with most of the "typical" IPA hops, I use about 1.5-2 lbs/bbl total.
For IPAs, and assuming that you're going for the biggest hop punch possible, there's no point whatsoever in anything other than a single, large hop charge at/after flameout. Sixty, 15, 10, 5, etc. min additions are useless. You'll get the bitterness, but you increasingly lose the oils. You get the best of both worlds with a single, large addition at/after flameout.