I am building an island upper cabinet that is 8' long (one piece), using maple veneer with MDF core. The cabinet has doors on the front and back, with a shared partition in between. The top, bottom and shared back are 3/4". For the back to bottom and back to top butt joints, is there any "rule of thumb" for the dowel spacing? I was thinking about maybe 3" OC for the first foot and then 6" OC in the middle. The way this cabinet is built, some or all of the faces are exposed, so conformat screws won't work.

BTW, I have a dowelmax.