This clone recipe has less than half the IBUs as the Lagunita's recipe. According to my calculations, the hops need to be increased significantly due to the high gravity of the wort. Lagunita's Brown Shugga has 52.1 IBUs. This clone has 25.3 IBUs. 
I use the formula provided by John Palmer for calculating IBUs:
IBU = AAU x U x 75 / Vrecipe
The Vrecipe variable is the volume of the recipe, in this case 6 gallons. The other variables need some explanation...
Let's start with the AAU (Alpha Acid Units). This clone recipe has three hop additions that provide bittering, so we'll just use those. I convert all the grams to ounces for this calculation:
44g Willamette 4.1% @ 90min :   AAU = 1.55oz x 4.1 = 6.4
10g Nuggets 12.7% @ 45min:     AAU = 0.35oz x 12.7 = 4.5 
31g Willamette 4.1% @ 45min:    AAU =  1.09oz x  4.1 = 4.5
To figure out U (utilization), you reference a chart that takes into account the gravity of the boiling wort, and the length of time the hops are boiled for. Higher gravity boils mean a lower IBU. So if you are making 6 gallons, but your boil is only with 4 gallons (because of the size of your boil pot), then your boiling wort will be even higher gravity than post-boil when you add water. But I'll be generous and assume you are boiling all 6 gallons, which will give you the lowest gravity for your boiling wort, and the highest IBU count. So your boiling wort is at 1.099 gravity.
Bittering hops are added at two different intervals, 90 minutes and 45 minutes. Using the 1.099 gravity you get the following U values:
For 90 minutes: 0.144
For 45 minutes: 0.123
Alright, let's plug in the numbers:
Williamette @90:  AAU = 6.4 x 0.144 x 75 / 6 = 11.5
Nuggets @45:  AAU = 4.5 x 0.123 x 75 / 6 = 6.9
Williamette @45:  AAU = 4.5 x 0.123 x 75 /6 = 6.9
Total IBU = 11.5 +6.9 + 6.9 = 25.3
References:
http://www.howtobrew.com/section1/chapter5-5.html