twirly, I feel your pain. Self-employment is tough and not the bed of roses it is made out to be.
I think you may need to include good and bad weeks and allow HMRC to decide what your average is.
As the business has only been going 7 months, they may expect your own earnings to be very low. All you can do is submit everything and they will get back to you.
Alas, guidelines are changing again and for 15/16 to qualify for WTC your profit will have to be 16 (or 24 if you're a couple) x NMW per week in order to qualify.
I was surprised to find we were part of an enquiry; it made me feel like we had done something wrong, yet I contacted HMRC with each and every single change of circumstance(s).
Ironically, my estimate of earnings was only £34.44 below what our actual earnings were. I'm sure that was one of the reasons our enquiry was concluded so soon.
And if I can give you one piece of advice, it would be to have an absolute handle on the financial side of the business - these checks (and further HMRC checks on your business) will become commonplace, especially with those whose earnings cannot simply be checked via PAYE.