The amount of SP does not determinate how much TPUs are issued.
Instead every time I issue new TPUs I manually set how much SP are backing them up - making sure that the ratio does not go lower.

So in this example after a year we have 80 tokens backed by 110 SP (1 TPU = 1.375 SP).
If I issue 10 new tokens, I need to add at least 13.75 SP to the pool backing the TPUs (to keep the previous ratio). If I issue 20 new tokens it needs to be 27.5 SP and so on.