You mentioned you want to do the headstock flat black. If you want the waterslide decal to be embedded in the finish and perfectly flush (non-raised), then you must layer multiple coats of clear over it and build up the clear around the decal. You don't want to do this with multiple coats of flat. I recently learned from a tech at Macbeath hardwood that any type of clear other than glossy is made non-glossy by way of an additive. Some additive=semi gloss, more additive=satin, still more additive=flat, etc. If you do multiple layers of non-gloss, then the additive can build up and cloud your finish. I was instructed to do all layers in gloss except for the final coat, and then do only the final coat in whatever sheen is desired or your final finish.
I apologize if this is old news for people experienced in applying finishes, but it was news for me.