I don't consider myself especially qualified to answer this, but since there are no other responses so far I'll venture an opinion.
How wide will probably be related to how far you intend to draw the bow and the draw weight you desire. I would keep it as wide as possible and leave the door open to narrow it while tillering or even after tillering but keeping in mind that weight would be reduced as the limb is narrowed.
I have wanted to try a juniper bow but have not found a piece of wood so far. See You are in Utah, so we are probably looking at very similar juniper.