The framing should be 12 1/2" from the center of the toilet drain so that it results in 12" off the finish wall. If you have not found the tub drain you should because any concrete breaking should come before the framing. 16"oc is good although you may be able to get by with 24" on your walls. 16" of course is better. Put in at least a 32" door and not a 24" and leave a couple of inches beyond that for framing.