There is no formula.

While there are people on FT who value their avios at 1p or 2p or perhaps more, the fact is that avios are worth different amounts to different people.

If this is the last flight you will ever take, why not burn all your avios on it as holding avios you will not use is a useless exercise? On the other hand, if you travel weekly, perhaps you want to look at relative costs.

You also need to look at flexibility and other ticket features.

This is no different than the people who ask whether F is worth it. It depends.