this is the formula I use (From TCJOH):
The variable is your Utilization. For a full boil between 1.040-1.070 I give 25% for 60 minutes. a little more than half that for 30 (13) and for 15 I give 10....The variable isnt linear so if 60=25 30 is a little higher than 12.5 and 15 is a little higher than 6.125. I just estimate and use 60=25, 30=13, 15=10, 10=5
so for your batch I get:
I wouldn't even factor in 1 minutes but maybe a few points.
It looks like they give you 30% for 60 min which is kind of high from what I've experienced and then another 7 for 1 minute:
I think your calculations are closer to what you'll actually get. The only way to know is to brew a few beers using whatever formula and compare the data to the actuall percieved bitterness. It's not very scientific but it works for me inso far as if I want a moderately bitter beer I use enough hops to my formula to give 35IBUs, or a really bitter beer I get +50. Compare your 35 or 50 to known commercial examples of those bitterness numbers as a litmus test.
(or just get ProMash or Beersmith but that takes the nerdiness out of the equation)