Hi all, my first post on here and it's regarding first wort hopping (again!). I know there are plenty of posts on here (and plenty of info on t'interweb) on the subject but I'm still somewhat confused about the amount of bitterness it imparts. From what I've read I get that I need to add 30%+ of my whole hop bill as FWH and that they need to be part of my late hops but I'm still stumped as to how many IBUs they're likely to give me?! I've read a lot of people on here saying they give similar IBUs as a 20 min addition but when I put them into my brewing software (I use BrewMate) as FWH it just increases the utilisation by about 10% which is also what I have read in other places when researching the technique. I'm looking for a nice <4% session beer with good body and some fruitiness using up the last of my stocks (literally everything in the recipe below!) and around 25IBUs but when I add in, what were a portion of my late hops as FWH, the IBUs BrewMate calculates them to impart are much higher than they would be as 20 min additions. I understand FWH gives the beer a much more 'rounded' bittereness but I still don't want THAT much bitterness in that style of beer. Any help or guidance on what to with my hop bill to incorporate some FWH (ie. reduce bittering addition etc) would be much appreciated as I want to give this technique a bash to see what (if anything) it adds to a beer. I'm relatively new to AG (only done 6 or so brews) but I do like to experiment

! Any thoughts on the rest of the recipe are also welcome, malt bill, mash temp etc.
Bubble and Squeak:
Original Gravity (OG): 1.039 (°P): 9.8
Final Gravity (FG): 1.010 (°P): 2.6
Alcohol (ABV): 3.83 %
Colour (SRM): 6.0 (EBC): 11.8
Bitterness (IBU): 38.9 (Average)
80% Maris Otter Malt
13.33% Munich I
6.67% Caramalt
0.5 g/L East Kent Golding (4.7% Alpha) @ 90 Minutes (First Wort)
1 g/L Willamette (3.6% Alpha) @ 90 Minutes (First Wort)
1 g/L Fuggles (6.1% Alpha) @ 60 Minutes (Boil)
1 g/L East Kent Golding (4.7% Alpha) @ 5 Minutes (Boil)
1 g/L Willamette (3.6% Alpha) @ 5 Minutes (Boil)
Single step Infusion at 66°C for 90 Minutes. Boil for 60 Minutes
Fermented at 20°C with Safale S-04
Many thanks and happy brewing!