These are some of the hardest questions founders face. And yet both have the same answer:
1/(1 - n)
Whenever you're trading stock in your company for anything, whether it's money or an employee or a deal with another company, the test for whether to do it is the same. You should give up n% of your company if what you trade it for improves your average outcome enough that the (100 - n)% you have left is worth more than the whole company was before.
Read more of this Paul Graham classic here.