Full Boil hop utilization

I have a Double hop IPA kit that I want make with a 5 gal. full boil. The calulator states it has a 29 IBU with a 2 gallon boil. This beer style calls for somewhere from 45-65 IBUs. The full boil puts it right where it belongs but I'm wondering if it will be too bitter. With a full boil, the calulator says I should cut all of the hop amounts in half.

Secondly, if I add the extract late in the boil (last 20 min), how does this affect the hop utilization? The recipe calls for 9.3 lbs of LME, would it help to use the 1 gal. to 1 lb. for the full 60 min and add the last 4 lbs. at 20 mins.

Lower gravities mean higher hop utilization, so a late add will boost the IBU even more. 45-65 IBU is correct for the style and shouldn't be a problem.

Right. Keeping your gravity lower (1 lb malt per 1 gal of water) will give you better hop bitterness extraction.