Since no one has replied, "I'll give it a shot."
I don't know what the "F" signifies. I thought there were only 21's and 22's.
Your 21 sounds very nice. Depending on condition is probably worth $1,500 or approximately that amount.
I had to add height to a front sight on a rifle. It turned out to be .30".
I believe you'll need to do so with your rifle. 18 MOA or a little over 18" at 100 yards would mean raising the front sight 18 MOA from the rear sight.
You'll need to measure the distance between the sights and calculate the height to be added. Say for instance the distance is one yard then the front sight would need to be raised 1/100 of 18" or .18".
If it is less than that -probably. The you take the distance between the sites in yards or fractions of a yard and divide by 100 where a MOA is only slightly greater than inches. Just a guess raise your site 1/8"
