I think the idea in NZ is that the layer always runs from 13.000' to FL150. In ISO conditions (QNH 1013) this means that the layer is 2000 feet thick. With a low QNH the layer will be thinner, and with a high QNH the layer will be thicker.
But you'd have a ridiculously low QNH to let FL150 be *lower* than 13.000', so that makes calculating the lowest available FL a doddle. Since it's always FL150.
On the other hand, you're essentially throwing away a whole 2000' (plus or minus some feet) of airspace. Simply because there's no formal way of altitude reporting when you're in the transition layer (you're "in transition" after all). So you can't fly there and be assured of vertical separation and such. If you have that 2000' to spare, fine. In congested airspace, airspace is actually a valuable resource.