Work out your start up costs, then add about 30%

I set up on the back of interest free credit cards so I had 6-12 months head start. If you have a good credit rating you don't have to go cap in hand to the bank who will then charge you double figure interest.
These deals are gradually becoming less generous. I still have 4k to pay off but can do a balance transfer for 2.5% then interest free for another year if need be.
I would say though the discipline of doing a full business plan is essential and update it as things change. I find accounts tedious, but understand it is important.