I need to fix some 3x2" battening to a solid wall (3 inch protruding from the wall). I was planning to use 8x120 frame fixings and reckon that this would give me around 46-50mm in the wall after countersinking. Is this satisfactory? Or should I go as deep as 10x135? Also if the 120 depth is ok would I be better using 10x120 instead? Many thanks.