When you use a solera, you typically glue the back on last. Until the back goes on, the sides will flex, allowing you to alter the neck angle pretty easily. Since the guitar is held to the solera when glueing on the back, the neck angle is locked in when the back goes on. The neck extension on the solera can be shaved down or shimmed up to change the angle that the neck makes to the body. In effect, the neck angle of your completed guitar is set by how far out of plane your neck extension is, with shims on the nut end of the extension giving you a neck 'set-back,' and a shaved down extension giving you a neck 'set-forward.' I happen to use a zero, or neutral neck angle, but that's just the way it all works out for me, given the doming I build into the top, the thickness of my fingerboard, and the height of my saddle above the soundboard.