Chunter,
Looks pretty good to me!
Do have to agree with David's idea -- if you can add another rail now, you'll be organized whenever you want to set up 60 round matches. (IMO, 60 is the way to go and I wouldn't think that you'd have to add any height to your stand to accommodate a third rail.)
Are you planning on pads for the feet or just set on the rails? Will you use backing plates, berms or...?