The numbers are pretty small. Pro rata doesn't apply to employee option pool dilution, so it's really probably only 5% of a round. If a VC says they will do an investment if they can own 20% but not 19% of a company, I believe they are lying.
Sam is right.... VCs put a line in the sand and everyone gets into a tizzy. Then you say to them "listen, I gave prorata to my early supporters and I intend to keep my word and reward them for their support."
The VC then has respect for the founder and say "OK, let's do it."
If they don't respect the prorata of the existing investors you need to ask yourself if this is the right VC to have as a partner. If they are so encouraging of you to screw your existing partners, how do you think they will treat you in a down market?
