Hop Utilization, Brewers Best Cream & Full Boil

I'm going to do a 5 gallon Brewer's Best Cream Ale extract kit with instructions for a 2.5 gallon boil as follows:

3.3 lb. Extra Light LME - 55 min boil
2.0 lb. Pilsen DME -- 15 min boil
1 lb. Corn sugar -- 15 min boil
1.25 oz. Willamette bittering hops 5.5% AA -- 55 min
1 oz. Willamette aroma hops 5.5% AA -- 10 min

I have the ability & desire to do a full boil with outdoor burner & wort chiller, so I want to do full boil. However, there is a note in the instructions indicating "IBUs will be higher with a larger boil." So, I've checked tastybrew.com's recipe calculator & come up with 37 IBUs (instructions show 16-18).

Can someone help me adjust the recipe (or the instructions) to more closely match the anticipated IBUs? Using the calculator I don't get in the range of IBUs until I get down to .5 oz bittering hops. Or should I just follow the partial boil instructions?

I'd probably just go with 1.00 ounce of bittering hops (5.5 AAUs) at 60 minutes. That's about what I use in my cream ale recipe, and it's just about right. I don't think 37 IBUs would be correct with only 1.25 ounce of 5.5 AAU hops for an hour, but I don't use that calculator.

Thanks!

I've also been curious about late extract addition & full boils: What effect does a full boil have on the late DME/corn sugar addition for this particular recipe?