Yep, you have to experiment with the OAL as the max OAL varies from rifle to rifle, and each one likes something different. There should be a section in your manual advising you on how to do that.
To adjust the seating die, you set the die for either crimping, or no crimp, and then you adjust the seater die plug on the top of the die, and keep adjusting and measuring until you get the OAL you want.