What are you using for a barrel preservative? Do you clean it out before shooting?
My Renegade shoots the first shot about 2" lower than the rest of the shots on a fouled bore. I assume once the bore is fouled the PRB fit is tighter and velocity increases a bit. Mine works out to a 50 yard zero on a clean bore and a 75 yard zero when fouled. Left/right is always good, just elevation is a bit different.
I've never tried, but bet I could use a bit thicker patch for my first load to get them closer, or perhaps a .535 RB first with the same patch and then go to .530.
But I'd rather just remember that I have a 50 yard zero when clean and a 75 fouled, much easier to me than to mess with different thickness patches or sizes of RB.
I use Barricade for storage, and when loading to go hunting I don't bother to remove it. This way I have bore protection until I shoot. I leave my rifle loaded during hunting season until I shoot. If I hunt in snow or rain I'll just pull the load with a ball puller rather than shooting it and having to do a full cleaning.
When I was using .015" cotton patches I did see quite a bit more difference between my 1st shot and the following fouled shots, but going to a .018" pillow ticking patch improved it quite a bit.
Idaholewis swears by popping a couple of 209 primers in his clean bore before loading so that the clean bore shot is the same as the following fouled shots. He did have to buy a different nipple set up to do this.