In ndebele stitching (and not only), the beads` height is often important. I love Czech beads (especially the 2+th grade) because you can find in one package beads from 1.1 mm to more than 1,8 mm high! This is convenient as you do not need to buy different packages. The main thing before work is to sort beads according to their height. I'll tell you how I do it.
You will need: flat smooth solid surface (I have for this tile) and combs-calibrators.
The comb is made from a piece of aluminum corner 1x1x12-15 cm, by gluing them on a superglue of 4 beads of the desired height:
Measuring the height of the bead height (in this case it is 1.3 mm).
Glue them with superglue to the corner.
You need to 4 beads. If less, the comb swings and the result is unstable.
Now it is enough to place the beads on the tile and hold them with a comb of the desired caliber, tightly pressing it to the tile. To be sure, check several times. It is better to sort the beads in small piles, otherwise the beads of the first row, gathered near the edge of the comb, do not give place for the next rows.
You get 2 piles of beads — from one side are beads less thick than a caliber of the comb, on the other — the one that are thicker.
I have 3 combs like this (two sizes for each corner).