Not that it matters to me as a retiree, but I also am convinced that it's up to 250% of the miles. To me the key is "up to 250% for every dollar you spend." What do you normally get for every dollar you spend? X miles, with X being dependent on the merchant. So an extra "250%", without explicitly qualifying it as a certain number of miles per dollar, has to be "up to 250%" of the miles.

Jim