Quote Originally Posted by Lyrinn View Post
Could be wrong but I vaguely remember from a long time ago the number being about 15%. The only difference between a WHM's Protect and a cross-classed Protect is Proshell, a WHM trait that also increases magic defence. So if a WHM casts it, it raises both physical and magical defence. If anyone else casts it, it raises only physical defence.
A WHM Protect will overwrite a Protect cast by a SCH. The buff icons are different. If a SCH casts protect after a WHM does, it's not applied (non-stacking.)

There's not a lot of value in cross-classing skills in FFXIV since the support traits don't go with it. Still 15% PDef is better than 0%