Late extract addition

I just brewed a the below recipe:

4lbs Light DME
1lb Rice Syrup Solids
1.25oz Willamette 60min
0.5oz Willamette 10min
2.5gal boil volume

My question is that I used BeerSmith to formulate this and I'm not sure if my IBU's are going to be accurate due to the fact that my first hop addition was in longer than 60 min because I added my extracts at 30min, but I waited for them to come back to a boil before I started my timer again. BeerSmith gives me an IBU of 15.1, but will this be higher since my hops were just actually steeping while I was waiting for it to return to a boil? If so...how should I account for this when formulating future recipe's with late extract additions?

I do the late-extract method. I have been using half of the extract in a recipe at the start of the boil and the other half after the heat is off. In BeerSmith you can have two lines for your extract in a recipe. For a recipe using 7 pounds I set two lines of 3.5 lbs each, and when you double click on it you can set one of these to "add after boil" (or something similar). It's a checkbox. It will change the IBU calculations drastically.

By doing this I don't pause a boil, I just add the remaining extract at the end of the boil and wait 10 minutes before using the chiller. It's how Palmer does it in How to Brew.

Thanks...I just redid my recipe with the same amount of hops that I used with the above method and it jumped it up about 5 IBU's. Looks like I will be over my target for this batch, but who knows...it may be decent.

So how can i calculate my IBU's if i am doing extract additions at 30min?