A lot of factors go into total timing. So much that you really can't go off of anyone else's setup. AF ratio, spark plug heat range, altitude you are running at, RPM range, piston type, cylinder head and combustion chamber efficiency, and on and on and on.

I have a 500 B on alky with 13.3 compression and domes that doesn't like any less than 38 degrees total. And the domes make it want more timing from my experiences and I have ran similar setups with flat tops that want lower total, maybe not like you are seeing but in the lower 30s anyway.. If she wants to run at 28, I would run her at 28!