Get on a call with them ASAP. Tell them you think that pro-rata is both what's FAIR and WHAT IS DONE 99.999% of the time in these circumstances-- and mention that you've done a lot of research. Say, "Guys, obviously any two of us can impose a decision on the third, but I trust you guys, so I'm assuming you're doing this because you think it's normal/fair. How about this: I pay for a few hours of a high-end startup lawyer's time and we can get a sense of what's standard-operation-procedure here is? It seems like we might have different opinions about what's fair. I'd propose we appeal to a knowledgeable 3rd party. Is that cool?" If they agree, add: "If this experienced startup lawyer says that pro-rata is what's done virtually all the time in these circumstances, can we agree to go that route?" And see what they say. Just keep saying you trust them and ask questions along these lines. Eventually they'll either agree or it'll come out that they feel that you haven't earned your 10%.
Understand: If things get ugly, they can fire you (wait-- are you vesting? If they fire you after 1.5 years, you lose most of your options). Once they fire you, they can dilute you and other people who are not with the company somewhat easily. Your job here is to be friends and to maintain/earn trust (and to not get taken advantage of!). NEVER THREATEN until there's nothing left to salvage. If you're a fabulous negotiator, you can HINT that you're willing (and financially able) to lawyer up.