I very rarely charge for anything like that.

With mates I just make sure that how long i spend on their car is directly proportionate to how big the fried breakfast is that they buy me at the end of it!

I tuned a blokes bike carbs (on kit car) the other day. Took me three jet changes to get spot on. I didn't know hime so charged him £20. When it came in it wouldn't rev past 3k and he had spent loads of time trying to sort it himself. He was so chuffed after driving it up the road that gave me £40 instead of £20.

I'm not a pro so don't charge pro money.