What is the very first thing you do before you make the first cast bullet?

You slug the barrel to measure the actual groove diameter, you never assume or just guess. Every barrel is a +or- specification.
About the only accuracy you will find is the groove diameter to .001 over the groove diameter. Different companies figure a bore rider differently from .004 to .006 or ever .008 difference, groove VS land diameter. if you measure your lands and groove after sluging, you get both numbers. You can size the front and the base both on a bore rider and most match shooters do exactly that.