Working out when mortgage would become 60% LTV

[FluidDruid]

We currently have a 69% LTV I was wondering whether working out when we would have 60% LTV and could get a better mortgage rate would help us to decide how long to fix for.

Is there an online calculator that could help with this or a way I could work it out myself?

The money saving supermarket one? By year, not month.
https://www.moneysavingexpert.com/mortgages/mortgage-rate-calculator/

You can also ask (and pay for) a revaluation if you think that property price increases may have changed you LTV