I'd definately suggest you find a gun shop that"ll let you try a couple mounts they might have in back. Don't need to have them all tight; just need to mock up a couple different ones to make sure. Get your scope on your gun and (while unloaded and pointed in a safe dierction..lol) find something to aim at. Close both your eyes and mount your gun to your shoulder and get comfortable behind the scope. When it feels "right" open your eyes and see if you are in the scope. You should'nt have to move your head up or down from there.
When i had mediums on my R-15 and did this i was looking at the top of the scope eyepiece. I literally had to cant my head sideways and "scrunch" my cheek into the stock to get centered. Was very un-natural and un-comfortable to say the least. definatelly give it a test run. You will be glad you did when you are out hunting. Who knows: mediums might be right for you. I just know that it is not usually the case, and i've mounted many a scopes for customers over the years.
Your Fastfire on the top is going to be very high and out of normal shooting position anyhow, so you might as well have your scope fit proper. The extra 1/8" or whatever it happens to be, will not much matter when you use the Fastfire.
Good luck and keep us posted.
Jim