Welcome to the world of .308 long range shooting. In standard conditions, and according to the book, you will be real close. 630 inches is only 63 MOA, that is about the minimum you need for 1000 with 308 and 20 MOA rail.
63, centered is 31.5 MOA up elevations available to you when you zero (usually you lose 2-5 MOA of that to zero at 100). With the 20 MOA base you gain another 20. So, best case you are looking at 50 MOA available to you to go from 100 to 1000 with the .308. In standard atmospheric conditions, and according to my "pretty accurate" software, you need 42 MOA with 168gr at 2600 muzzle velocities. You have, at MOST 50 available to you, thats cutting it close. Throw in any variables, like higher humidity than standard, more up MOA needed to zero at 100, colder weather, BC's that are not as high as manufacturer claims, and you are out of UP (like you ran into).
I consider 70 MOA a minimum with a 20 MOA rail, but prefer 80 MOA. This is a big plus for 30mm tubes, they tend to have more adjustments. the Leupold Mk4 has about 95 MOA. That cheapo bushnell 3200 10x tactical scope you can buy for $180 actually has 100 MOA. There are a few others... but not many.
You can add shims under ther rear of your rail, but I don't recommend that. If you have mildots, you can learn your holds also at that range.... of course, you can buy a new scope to
MEL