Hi DH, 3/8 of an inch seems a little high BUT the deep seal wax gasket by Hercules
(horn type) should have no problem taking up for this space.
You may have a rocking motion from this space. You could possibly skim coat this slab to raise the concrete to a proper level
About the offset floor flange I use them a lot and never had a call back BUT not every code allows them.
The offset floor flange I use is heavy brass type lead wiped (soldered) to a lead bend.
If your working with lead you should honestly try to lower the flange and /or grind the slab level for a proper connection to the waste bend and do away with this 3/8 height difference so you know you will have a water and gas tight connection.