You want to make a half rectangle triangle block - as opposed to a half square triangle block.
You can't use the same method as the half rectangles triangles are not interchangeable the way the parts of a HST's are.
You would have to take your two rectangles (oversized) and cut them in half on the diagonal. Then match up the one triangle A from fabric A with triangle B from fabric B.
If you're making a bunch of these blocks, make sure your fabrics are ALL facing the same way (right sides up) when you make the diagonal cut, and always make the diagonal cut in the same manner - lower left to upper right (or visa versa).
Just be consistent.
I would imagine the math to figure out the size of the starting rectangles is similar to that of HST's.
So if you want a finished measurement of 8x10, then I'd start with a minimum of a 9.5 x 11.5 rectangle, make the HRT's as described above, and then trim down to the 8.5 x 10.5 unfinished.