It's flat. But the Earvana's I have have excess height on the bottom. Note too that there are two different radii (radiuses for you hillbilli) available for 1 5/8 and 1 11/16". Warmoth compound radius is 10"-16". Earvana 1 5/8" comes in 7.25" and 12", 1 11/16" comes in 9.5" or 12".
I went with a 1 11/16" neck, Compound radius, and put a 9.5" radius nut on it, and filed the bottom of the nut (the slot portion) down until the shelf just kissed the fingerboard, then I took the bottom of the shelf down to minimize the very slight gap in the middle. Just because I was obsessing about it. The difference is very very very slight for the arc width that the nut occupies.
If you're planning to put an Earvana drop in I think you have a couple choices
1) Order a 1 11/16" neck with a 9.5" radius
2) Order a 1 5/8" neck, 12" radius neck and use the 12" radius nut.
3) Order a 1 11/16" neck with compound radius and use the 9.5" radius nut
