Creative Grids Itty Bitty Eights rulers might also help. Kind of like using a 1/4" mark and splitting it, you would have 1/8" marks you could split. I almost think some of them have a 3/16" mark on them, but am not home until later in April so cannot check for you.
My biggest tip when making tiny little squares from small pieces is to starch (or starch alternative such as Best Press) before cutting. I found when doing 3 or 6 inch blocks with 30 or 40 pieces - and yes, you will have blocks with that many pieces - the pressing before cutting really helped.