Generally I would suggest brewing a couple more kits before you start moving into coming up with your own recipe. But with that said I do love Citra
So what amounts to sub really depend on a couple factors. The starting gravity of your IPA / amount of extract and or grains, the hop schedule and alpha acid percentage of your kit hops and how much your going to use for a boil volume (based on the kit instructions).
So IPAs generally have a decent range in starting gravity this will effect how much IBUs you'll want to get out of your hops. Your on the right track with knowing the alpha acid percentages of your existing and substitution hops. There are a number of brewing apps for ios / droid and web sites that allow you to create recipes. Key in the existing information for the base kit and hops then start working the numbers with your Citra.
I would follow the kit recipe to the T and sub out the hops based on what your seeing in the brewing software, try to hit close to the same IBUs, Citra is a high alpha acid hop so I think you're probably going to only use about 1 - 1.5 oz bittering and probably 2 oz at the end of the boil. I suggest late additions of 10, 5 and flame out at .5 oz each. When you dry hop put your leaf hops in a hop bag.