I've never made a jelly roll quilt, but if 40 strips gives you 1600" sewn end to end, then 22 strips will give you 880" sewn end to end. When you start dividing it up then the first strip will be 4.5"x440", then 8.5x220", then 16.5"x110", then 32.5"x55".
I guess in order to figure out how many strips you will need, figure out how long you want it, then start multiplying in the other direction. So, if you want it 60x80 . . . . . . . 80 inches long, x 30 strips wide, 80 inches long would be about 2 wof. So, 30 strips wide x 2 strips would be about 60 strips.
I may be blowing smoke here, but I think that is what I would do. Maybe someone that has made jelly rolls could figure out a better way? (I may start cutting strips just to see if this will work.)
I do believe you need to cut off 18" and discard otherwise your seams will be lining up.