🚀 go-pugleaf

I'm running Frame 7.0p492 on Windows NT4.0. I'm trying to create a 2-up page layout that allows me to create 5.5x8.5 booklets that I can easily print duplex on 8.5x11 paper in landscape. 

The problem is how to create the automatic page numbering. In a 10-page booklet, the 1st printed page has pg 1 on the right and pg 10 on the left; 2nd printed page = pg 9 right, pg 2 left; 3rd = pg 3 right, pg 8 left; 4th = pg 7 right, pg 4 left; 5th = pg 5 right, pg 6 left. 

For pages 1-5, I can just insert the system variable $curpagenum. But for pages 6-10, that doesn't work. I really need a "countdown" variable that starts at twice the number of printed sheets. A simple equation, ?=((2x$lastpagenum)+1)-$curpagenum would work (you'd get a nice countdown as the $curpagenum value increases), but Frame doesn't allow the use of system variables in equations. 

Any ideas?