You use a set of calibers to measure your OAL from your rim to the tip of your bullet. Do you have calibers?
You move the seater die finger adjustment to adjust the seating position of the bullet. You seat the bullet out starting high and measure with calibers. You start out your seater die with finger screw screwed out several turns(this means your OAL is going to be really long at first). And you seat your bullet. Take your loaded round out of the die, and you then measure with calibers. If you need to adjust, you turn the finger screw in more or less. You can't setup the seater die to hit exact OAL right from the getgo.