Quarter sawn for your backing is fine as long as the grain is good and straight. I've never used oak for the core, or ipe at all for that matter, but I can give you the widths from a hickory/cherry/osage tri-lam I made that I really like. I made the hickory back and cherry core both 1/8" at the center and tapered them to 1/16" from just past the fades. The osage belly was 5/16". Just had to do a little rasping to get the tiller right. It's 66" tip to tip, 1 3/8" at the fades and 47# DW. Hope this helps and good luck.