The screen that gives you the price and mileage amount is the final purchase screen. Short of having you "hold" miles (like holding space on a flight) with the dropdown, there's no way to do that. With only 10M eligible for the top bonus, there are going to be a LOT of people there trying to buy.

20 people are able to buy the full 500k.

The bovine and I are each talking about buying the full amount. That would be 10% of the available bonus pool at 115%.

I don't think that's going to happen, but there's serious consideration.

Not that 1.1cpm is much worse than 1.0cpm, which means there's 110M available.

But I suspect you're going to have to make the purchase with no idea whether you're getting 115%, 90%, or 65%. I'd bet that if you do it by 1015 EDT, it'll be at least 90%. But I could see the 115% running out by 10:00:05.