I build up my Lufthansa Miles and wait until the cheapest fare is at least 75% more than the tax to pay if I redeem the miles for a flight. It usually happens that at some time of the year the standard fare for my regular route becomes very expensive such that the tax paid using miles may be half the standard fare - or I need to go somewhere at short notice and the price is simply very high for that reason.
It is surprising though that the tax for a miles flight appears to be a bit more than for a standard fare.