so if I look one year back dificulty increase by arround 60% ... ? so if same thing happens till next year my daily earnings will drop from 20$ to 8$ if btc price not change ... can I calculate something like that ? will be this calculation close ?
This calculation is wrong. If difficulty increase by 120%, miner still mine some coin, but you have fixed fee of 15Th x 1.2$=18$.
So if the 15Th earn less than 18$ per day for 20 consecutive days the contracts will terminate.
15Th earn 0.1146 btc, with 60% diff increase will earn 0.071625 btc (for 30% increase divide the btc by 1.3, for 120% increase divide the btc by 2.2),
if BTC will be at 240$ in 6 month -> 17.19$, so minus fee of 18$... no profit ! (contract will terminate)
if BTC will be at 300$ in 6 month -> 21.49$, so minus fee of 18$=3.49$
if BTC will be at 500$ in 6 month ->35.81$, minus fee = 17.81$
For this kind of contract the BTC price if very important (because of the fixed fee). Sorry for my bad english.