If you were making it out of osage, I'd say 66" might work but I'd still be inclined to go 68". Making it out of oak though, if it was me, I'd be planning it for 72" for a 30" pull and a fairly wide, flat limb cross-section, especially if you're using red oak. It is pretty susceptable to compression fractures on the belly.
I've never used white oak, so I don't know if it's the same way or not. And keep in mind that I'm pretty conservative when it comes to bow design. I'd rather not have one up and whack me in the head because I made it too short, and I've never had one break because I made it too long.