Improving on the first brew

So with a fair bit of the first brew drank (and an extract christmas brew bottled), I have identified a few points for improving my first beer. The two key areas being carbonation and head retention.

Carbonation I have sorted, I am going to batch-prime with an appropriate amount of sugar for a wheat beer.

Head retention - The increased carbonation from batch-priming will help with this. I'll also be adding 1kg wheat DME to the kit along with 250g amber malt and 200g Carared grain for steeping. I'm going to boil the steeped solution to sterilise it and was thinking a 3L boil in total for 30 minutes using the 250g of amber DME to bring the gravity up to 1.040, dissolve the remaining DME towards the end, add the kit and then top up with cold water to 21L.

Now my one and only question is I was going to throw in 11% AA Centennial hops to add something different to the mix and boost the IBUs a bit. Would 20g suffice? I want to put my own stamp on this but without killing the standard weissbeer flavours. Cheers and beers!

Ingredients used

1.8kg Muntons Wheat beer kit
1kg dry wheat malt
250g amber malt
150g carared
Xg of Centennial hops
WLP300- Hefeweizen yeast

It depends on how much hop flavor you want. It looks as if you're going for a very light wheat beer and 20 g of Centennial will give it a moderate but pleasant bitter bite (assuming you add them for 60 mins--not sure on the exact IBUs). Id add half an oz of hallertau in the last 5 mins for aroma (if you like that). If you have a target IBU in mind, i'd use one of the free recipe making sources like "beer calculus" on hopville.com to get a better idea.

Very light? I'm aiming for ~5%. The 1.040 gravity is just for better hop utilisation during the boil. Cheers for the advice though, think I'll go for 1 oz of Centennial at 40 mins to get a flavour/bitterness balance. Cheers