Buz-- leave some "wiggle room" for the eventuality that the weather be great and allow you to start at SJPD. I have been monitoring weather along the
Camino Frances for the last 4-5 years and April is trending with much better weather than May. If anything consider starting SJPDP-Orisson, and if weather /Route Napoleon is in bad shape, taxi to Roncesvalles (Express Bouricott) and move on from there. I am a huge fan of starting at SJPDP, if anything because there is such an energy and pilgrimage mystique at this beautiful village. Buen Camino.