If I'm not mistaken, YC gets pro-rata as preferred shareholders, so their dilution is not any different from everyone else in that class.
Pro-rata is in theory the right (but not the obligation) to buy back in, at the new price, to get back to your prior ownership level.
In practice however, pro-rata is a firmly stated polite request to the next round lead investor to let you do so. The real determinant of whether you get to use the pro-rata right is whether your next round lead investor is OK with it (and whether you want to try to block that next round).