Basically.
EATX is kind of a bastardization of the ATX formfactor with allowances for boards matching up to the SSI-EEB length and width, but not mounting points.
The best way to account for it is to base your measurements off of the standard ATX mounts with extra room past the right most standoffs for whatever extra width the board you have in mind is. For example, I have an SSI-CEB (12"x10") motherboard and an SSI-EEB (12"x14") board. EATX isn't a consistent length, but a range anywhere between standard ATX all the way out to SSI-EEB. But whatever board you're looking at should have its exact measurements on the manufacturer page. Or you could play it safe and just make enough room for a full SSI-EEB.
A real good place to start would be basing your measurements off of the Cerberus X, although a heads-up from someone that learned the hard way, despite their stated measurements, it was about 4mm too shallow to fit the full horizontal length.