I'm pretty new at this, but so far, all I've made have been reflex/deflex long bows. I've put in an awful lot more deflex, though, than I see in your bow.
I've found what Semo says about tillering R/D bows is very true -- you can keep plugging along with no bend at mid-limb and then, a second or two too long on the belt sander or a scrape or two too many, they're bending at mid-limb way below your target weight. Take it slow and exercise the bow a lot each time you take off some wood.
If your deflex is extreme (and it isn't on your bow), you're not likely to ever get a perfectly elliptical "D" shape. With the very negligible deflex on your bow, however, I'd do as Pearl suggests and tiller it just like a straight longbow -- start at the end and work your way toward the fades.
Good luck. I've always been a recurve guy, but I've found that these long, thin, board-bow longbows are smooth to draw and shoot like whips.