I don't know how much this really affects dilution. Without YC doing this, your shares would represent 80% of their previous value (X + .2 Z = Z). If YC adds so it maintains 7%, you would have 78%(X + .2 Z + (.07 Z - 0.07 X) = Z, .93 X = .73 Z)
So if you have 30% right now, you would have either 24% without YC taking part or 23.4% if they do.
So if you have 30% right now, you would have either 24% without YC taking part or 23.4% if they do.