Using 12-3 with ground to wire a new light
The circuit layout is sound.  A small point, I don't think you mean to actually use a pigtail (short additional jumper wire) in the junction box, just connect the light cable to the switch loop in the junction box.  

It sounds like you're just going to have one junction box for all the lights and switches in the attic, rather than one per room.  That is OK as long as the junction box is large enough to house all the splices - you'd need to do the box fill calculations to determine.  

Your description is kind of a star layout from a single junction box.  You more often see more of a daisy chain layout where the source power feeds the box for the first light for each room, and the switch loops for that room come from that box.  That will usually use less cable and avoids the big junction box.  

The star layout may have advantages too, for maintenance, troubleshooting, future additions, etc., but only if it's a BIG box easy to work in, not a disastrous tangle of unlabeled splices.