Ridge's "dip rod" test gives you a COAL - you mark the rod when inserted down the barrel and held against an empty bolt face, then you mark the rod while inserted and held against the tip of a bullet, held in against the rifling with a pencil. The measurement between those two marks will tell you the distance between your closed bolt and the tip of your bullet (Max COAL to kiss lands) if you were to seat the bullet to the lands.

You do not need to make any correction at all for base thickness - your only adjustment to that measurement is your desired bullet jump.

The Curved hornady COAL gauge is designed for levers and autos, or basically any other model that doesn't allow ready access to the boreline from the rear. I've used a straight model in AR's in the past, just remove the upper from the lower.

For me, the Hornady OAL gauges are useless tools. For it to be useful requires far more work and calculation than it is worth, and takes longer than doing it the old fashioned way.

You'll spend a lot more time calibrating your hornady modified cases to your properly sized brass then using the gauge and punching numbers in your calculator than it would take to have simply done Ridge's dip test or my smoking test. Waste of money.