Hi Terry,
When the koma is in place, the strings are close to the sawari tani (the ridge), but not actually touching it. (otherwise, it would buzz… and not in the right way )
The height location of the kamigoma (brass nut) depends on the angle of the tenjin to the sao. If the tenjin is pitched up, the string action would be higher and thus the kamigoma would be placed lower to accommodate. In Ale’s case, his kamigoma is quite a bit lower, which would mean that his tenjin is probably angled higher.
If the tenjin is angled downwards, the string action would be lower and thus the kamigoma would be placed higher.
There’s nothing really set in stone on the matter, it’s all dependent on your string action preferences.