For SCA fighting, you should probably start with at least 8 ounce leather, which the hardening process will thicken to about 10 ounce. 12-14 ounce is better--but harder to stretch over forms. In general, you will want thicker leather over vulnerable points such as knees and elbows--which means either starting with thicker leather or leaving your leather in the hot water longer to get more shrinkage.
Sometimes something will go wrong; you pull the piece of leather intended for a bazuband out of the water only to discover that it has shrunk too much to fit your form, or that a thin section you didn't notice has shriveled up, ruining the piece. All is not lost. Flatten the piece out and cut it into lamellae. Enough mistakes and you have a free klibanian.
One minor problem with the process is that the leather gradually turns the water you are using brown. For a simple project, such as making lamellae, this may be tolerable; you can stir the piece of leather to the top of the pot every ten seconds or so to check its condition. For something more complicated, such as a bazuband or greaves, you may want to heat fresh water for each piece--which is a nuisance but makes it easier to see what you are doing.