I know I might be asking too many questions about your seat but with the price I had to pay for leather.
The seatbase pattern did you put the paper around the edge on the bottom or drape the paper over the foam.Sorry I just don't wanna cut my leather too small.
Normally my grumpier self would say, " How is it that because you bought pricey leather I have to answer questions?"
But I am not too grumpy right now.
First I must say that this seat was the first that I have ever done. I am not an upholsterer or leather worker.
I have never seen a seat done the way I did mine.
I just thought it through and figured I could get a good result doing it in the manner that I did.
Your results may vary.
I placed the seat pan on the tracing paper and drew a perimeter line slightly larger than the pan.
This was done to give a little extra material to account for the curvature in the seat pan.
The amount that the pattern was slightly larger than the seat base will vary depending on how thick the foam is, how it is installed and trimmed, and how your particular seat pan curves.
How thick your leather is will also count when you factor in how stretchy it is or isn't.
This is because getting the pan slid inside the two pieces requires some tugging.
The shape of the seat pan is also a factor in how easy it is to get inside.
I can also see that if the pattern is drawn to tight to the seat pan it would make a finished product that is really difficult to get the pan inside.
As it was with mine I had to make a couple of small cuts to open the underside panel to get the pan slid inside without tearing the leather by stretching it open.
Maybe try making a tester piece using some old denim blue jeans, or go to the local Salvation Army store and buy an old leather jacket or ladies handbag for cheap to get something to experiment with.
Good luck.