I wouldn't do the calculation of the yardage for precuts based on square inches. I would do it based on the number of pieces that are cut from one WOF. So, for a jelly roll, you get one strip from one WOF. I would assume that the useable WOF is 40", so for a layer cake of 10" squares, you can get 4 pieces from 1 WOF, and for a charm pack of 5" squares, you can get 8 squares from 1 WOF.
The next part is how many strips/pieces do you get in your precut pack. I've seen mostly 20-40, but others have reported other numbers of pieces, so you have to adjust for that.
40 2.5" strips is 40 *2.5" = 100"
divide by 36" for yards or 39.4" for meters
100"/36"/yd = 2.8 yd or 100"/39.4" = 2.6 meters
40 10" squares means 40/4/WOF or 10 strips at WOF 10 10" strips = 100", so same as above for 2.5" strips
40 5" squares means 40/8/WOF or 5 5" strips at WOF 5 x 5" strips = 25" of fabric
25"/36"/yd = 0.70 yards or 25"/39.4"/m = 0.64 meters
And you probably pay a premium for the pre-cuts, but you don't have to cut them yourself.