Ready for more math or math explanation?
To figure out the percentage which you already knew was "about half" the mathematical approach is to divide the number of blocks you want as opposed to the number of blocks designed -- 25 by 49 or 25/49 which gives us 0.510, so we know that .6 will work for the reduction without as much extra as my .75.
All you have to do is multiply the yardage requirements by .6 -- is easy with whole yards (like I used 2 and 3) above. Quarter yards are .25, .5 or .75. Thirds in decimal equivalents are .33 (1/3) and .66 (2/3). In your it gets harder example, 1-2/3 yards equal 1.66, 1.66*.6 = 0.996, or round up to 1 yard
Since the design is actually flat set 12" blocks and you want it to be 5x5 blocks that makes the design work out nicely. Look at the upper left corner to see the block -- it ends with dark corners. We'll call that Block A. Block B next to it has white corners. You will need 13 Block A and 12 Block B.
Isn't it amazing how color values change the design?? I mentioned in the post above that the center large prints are the same in both blocks (maybe even fussy cut a bit) -- the star points are also the same color in both blocks. But for me, they pretty much disappear in Block A but come back strong in Block B when set against dark fabric.
I hope you will post pictures of your project!