On most rifles even with a 20MOA base you will still be between.15ish to as much as 40 MOA off the bottom of the travel . My 308 , Rem 700 , with a 20 MOA Badger base is 35 MOA off the bottom of travel on a 3.5-15 Nightforce .

You cant count on being right off the bottom of travel because manufacturing tolerances vary and stack . The math works ok for doing the rough figuring but really the only way to know for sure what you end up with is to mount it up and see .

If the length of pull and cheek piece height is adjusted properly neck strain is a non issue unless your neck is jacked up to begin with . Only one of my bolt guns has a nonadjustable cheek piece and it will be the only one the rest are all adjustable .

On a 338LM class gun I'd look for a 30 or 40 MOA base to get the most usable elevation possible . Your data past about 650-700 yards will vary way more than closer with changes in atmospheric conditions and even more so past 1000 . It just makes sense to get a scope with the most usable travel to take advantage of the guns capabilities . With factory 338LM being 6 plus dollars a shot there is nothing cheap in the endeavor so it makes sense to get good stuff up front . It makes the learning curve shorter and a little less steep .