You can probably tweak the recipe in a recipe calculator to get the IBUs back where you wanted them to be.
But, unless your recipe is for a bigger beer (OG > 1.055) I don't think it is necessary.
When playing with ProMash one day I found that my IBUs pretty much stayed the same as long as my OG was under 1.055. Once I got above 1.055, the IBUs started to drop a little.
The easiest way to do this is to take whatever calculator you used to design the recipe and go back into it and start lowering your efficiency until you see the OG that you actually achieved. Then, back of on the bittering hops in your recipe until you get what you want.