Help... plan to brew today...

I'm typing up my recipe for today's brew and am getting a little confused. Would like any help I can get. I'm using BeerTarget and am getting a little thrown off by the results. I have a 5 gallon 1 quart pot, so a 5 gallon boil is a little out of my boil range. So I would say my target boil size is around 3 gallons. I plan to steep my specialty grains for 30 min. in 2.5 quarts, then fill to 3 gallons to bring to a boil. Add DME, stir, rolling boil again, then add fuggle hops for 45 min., then add the Corn Syrup, Cocoa, etc..., then add the Kent Gold. Hops in the last 15 min. of the boil, and do a late addition on my LME. However, when I change my boil size to 3 gallons (batch size remaining at 5 gallons) in BrewTarget, it dramatically changes my results to where I'm questioning if I'm doing this right. My recipe is listed below. My main question is, I don't want this beer to be too bitter... 30-35 IBU range is good with me... BrewTarget lists it as 63.2 when I change the boil size to 3.5 gallons from the 5 gallon boil size default... is this accurate?

Anticipated OG: 1.051
Anticipated FG: 1.013
Anticipated SRM: 29.7
Anticipated IBU: 28-34.9

1.0 lbs. Briess Golden Light DME
4.0 lbs. Northern Brewer Organic Light LME (Late Addition)
0.5 lbs. Dark Corn Syrup
0.8 lbs. Briess Caramel Crystal Malt (60L)
0.75 lbs. Simpsons Chocolate Malt
12.0 oz. Lactose (15 min.)
6.0 oz. Cocoa Powder (15)
.33 oz. Liquid Chocolate Extract (Secondary)
½ tsp. Irish Moss (15 min)
½ tsp. Yeast Nutrients (15 min)
1 oz. Fuggles Hops (60 min)
1 oz. Kent Golding Hops (15 min)
Wyeast 1318 London Ale III
3.8 oz. Corn Sugar (Priming)

A smaller boil volume will decrease the IBUs you get out of your hops a little. I don't know BeerTarget specifically, but BeerSmith has independent fields for boil volume and batch size. Make sure your batch size is set to 5 gallons.