this will not work for your rectangles, but I haven't seen anyone mention the quilter's magic number - 1.414 - to use in resizing an equilateral triangle - meaning a triangle that has two sides equal. The link below takes you to a Martingale blog where they describe it more fully, but, basically, if you want to resize a triangle with two equal sides, determine the length of the short/equal sides of the triangle then multiply that number by 1.414 to get the length of the third side. You'll have to round some numbers off so do them to the nearest eighth of an inch. Check out the link - It's a number worth remembering.
http://blog.shopmartingale.com/quilt...ing-triangles/