Technically, Tinseth's hop util. equation only takes into account boil gravity and time. The IBU equation is just IBU = AAU * Util. * 75(Constant) / (Batch Size in Gallons), so I think scaling up should simply be multiplicative. For example, here are some of my numbers from a previous brew:
1 oz UK Kent Goldings (5.8% alpha) (30 min)
Boil Grav. = 1.090
Assuming 10% extra util. for pelletized hops
Util(30) = (1.65×(.000125^.090))×((1−(e^(−.04×30) ) )÷4.15)
= 0.123741 * 1.1 (pelletized) = 0.136115
AAU (Kent 30) = 1 oz * 5.8 alpha = 5.8 AAUs
IBU (Kent 30) = 5.8 AAUs * 0.136115 util * 75 / 5.0 gal. batch size = 11.842
So scaling *up* to 10 gallons should be as simple as doubling the hops in this case, or scaling by whatever factor applies.
Brundoggie, for curiosity's sake, can you post the recipe you're working with?
It's a Fire Rock Clone:
Recipe Type: All Grain
Yeast: Kolsch
Yeast Starter: Slurry
Batch Size (Gallons): 11.25
Original Gravity: 1.053
Final Gravity: 1.014
IBU: 35.2
Boiling Time (Minutes): 90
Color: 6.7
Primary Fermentation (# of Days & Temp): 14 at 68 degrees
Additional Fermentation: Cold conditioned for 10 days
Secondary Fermentation (# of Days & Temp): Straigh to keg
Recipe Specifications
--------------------------
Batch Size: 11.00 gal
Boil Size: 14.40 gal
Estimated OG: 1.053 SG
Estimated Color: 6.7 SRM
Estimated IBU: 35.2 IBU
Brewhouse Efficiency: 72.0 %
Boil Time: 90 Minutes
Ingredients:
------------
18.00 lb Pale Malt, Maris Otter (3.0 SRM)
2.00 lb Munich Malt (9.0 SRM)
1.00 lb Honey Malt (25.0 SRM)
0.50 lb Cara-Pils/Dextrine (2.0 SRM)
1.00 oz Centennial [9.50%] (60 min)
0.50 oz Cascade [7.80%] (40 min)
0.50 oz Cascade [7.80%] (30 min)
0.50 oz Cascade [7.80%] (20 min)
0.50 oz Cascade [7.80%] (10 min)
0.50 oz Mt Hood [3.70%] (5 min)
1 Pkgs German Ale/Kolsch (White Labs #WLP029) Yeast-Ale or Safale04 of WLP051
Dry hop with ½ Oz of Centennial per 5-gallons for 5-7 days.
Mashed at 154.