Richy,
I know everyone says business and mates don't mix but I set up a business with my best mate.
And when I say best mates, we really were - spent 24/7 with each other, never ever argued or fell out always had a great laugh.
We broke the business up 7 years ago and guess when was the last time I spoke to him.......... you guessed it 7 years ago!
My best advice is really really dont do it. As you said he's hands on and you prefer the admin side - or as he'll see it a couple of years down the line, he does all the hard work and you just sit on your arse & resentment will creep in!
But I suspect you've already decided and think that it'll be different for you 2, so the best thing I can recommend is to give him nothing now (pertcentage wise) but put a value on any new business he brings in i.e. 2x a clean and allow him to buy in this way.
Even with this I doubt it'll work out, but I hope it does and wish you the best of luck