You are correct. The divider must be concave to match the convex valve

Extensive use of a common childs toy is used to check correct shape and clearance.

It's a good bit closer than that, but I'm not telling how much

This is what the bottom of the WG valve should look like if it's done correctly. You can see there is almost no cross contamination between paired cylinders. This valve was removed after several miles of casual driving to show this effect. If it had been removed after WOT you would not see this line due to the valve being off the seat.

 

Please tell me the distance, looks like you got it perfect!