I may be in over my head

The wife and I are renovating our first house and our first order of business is to rip up the carpet and put down hardwood. Manufacturer recommends 3/16 flatness over 8’ radius which feels like a pretty tight tolerance but we’re trying to hit it. The master bedroom is 21’ wide and showing roughly 7/8” variation from the highest spot to the lowest (that’s the only room I’ve checked so far). My question is what is the best way to handle getting it flat, am I supposed to just keep sanding the high spots until they’re low or are there other methods aside from ripping up the subfloor and replacing it.