The question of how much to fund by is the same as the question of whether to fund. As a VC you have to trust the companies you fund to spend your money wisely and give them a long leash, letting them spend their money on things you think are stupid - otherwise you're not a VC at all, you're more like the R&D department of a large corporation (which could be a reasonable and profitable thing to be, but there's presumably a reason VC has grown so much and corporate R&D has shrunk).