http://tradgang.com/noncgi/ultimatebb.php?ubb=get_topic;f=125;t=001047#000000 This is a link to the thread about how to make a tillering gizmo. It is basically idiot proof, and will get you a perfectly circular tiller without taking off any more wood than necessary.
I have some advice for the riser on your bow. I am not sure if you are familiar with the terms "flares" and "fades". The flares are the transition from handle to limb in width. The fades are the transition from handle to limb in depth(thickness). (both the handle and the flares/fades are included in the riser) On your bow it appears that the flares happen first, and then the fades. If you place the flare and fade of a limb into the same area, you get a smoother, shorter, fade/flare section. This eliminates a lot of the problems people have with bows breaking at the fades. It also extends the length of the working limb, making defects in tillering(like a hinge)less critical. (the smoother fadeout also reduces the chance of a hinge at the end of the fade)Now on this bow, I am not sure if you want to make more changes or if you want to just finish it up. But on your next bow I suggest you try these changes.
If you do decide to make changes to this bow, extending the working limb will reduce the stress on it, allowing for changes to be made to bring the draw weight back up. For example you could reflex the bow, and/or shorten it.
If you just decide to retiller the bow and not change the flare/fade layout, you might want to consider reflexing the limbs and heat tempering the belly.(to counteract draw weight lost in tillering) This can be done with any kind of a form(straight board with shims, just a straight board to bring the bow back straight, a fancy form, etc) and a heat gun, or a long bed of coals. When heat tempering only heat the belly of the bow, and heat until the wood changes color, not until it blackens. Also if you do not want to extend the length of the working limb, I suggest you change your fadeouts, so that they start at the end of your grip and end in it's current position. This will help when tillering because it helps eliminate hinges near the fades.
If you decide to follow any of my suggestions please pm me, and I can provide more detailed explanations as well as pictures of what I mean.