I'm not an expert brew chemist or software guru. So please consider the source as you make your decision as to how you will proceed.
I FWH every batch because I buy hops in small quantities as I brew. I want to get the best "Bang for the Buck" with my hops as at .oz quantities from the LHBS they are expensive. Haveing read Papazian, Plamer, More, & Daniels, and many other articles, I believe that the basic IBU calculation is based on the isomerization of the Alpha Acids accomplished by boiling. Other factors such as steeping or dry hopping are usually omitted because while they contribute oils & resins that give flavor and aroma, they are not boiled, therefore they do not contribute to the IBU's bittering the beer.
As I understand Mash Hopping and First Wort Hopping, since they create a long period of contact with the wort at elevated temps (not boiling but significantly elevated) they soften the resins & oils allowing them to be more easily and effectively isomerized during the boil. Each "Hop Guru" has a slightly different take on this so thier calculation are different. Usully they eventully lump it into positive percentage of the IBU contribution. So as mentioned above, steeping and dry hopping are NOT followed closely by a boil, they do not make an IBU contribution.
Because I'm just a small batch homebrewer, I have no way to cost effectively test for IBU much less account for an actual percentage increase based on MH or FWH. I just assume, becasue people much smarter than me say they do, MH and FWH make an aproximate 10% increase.
Clear as mud??
I think, just as there are significant differences between the "Hop Guru's" like Garetz, Rager, and Tinseth on Utilization, us everyday joe's just need to pick one and stay with it.
Ray Daniel's view seems elegant to me. He says he thinks Rager was a pessimist, Garetz was an Optimist, and Tinseth was somewhere in between. So he picked Tinseth. So do I based on his middle of the road opinion. Further, he feels most of us can not precieve a difference in bittering of less than 5 IBU. So where does this leave us?
I picked one calculation method, Tinseth. I accept Brad Smith's (author of BeerSmith) 10% increase in contribution for MH or FWH and then quit worring about it, as most of these improvements are less than 5 IBU'sanyway!!! I now just leave the calcs setup the same way every time and brew to please the Princess and myself!! By keeping the calculations the same every time, I have the best chance of repeating a brew that we really liked.
Like I said, Clear as mud!!