In my opinion, 1600-1800 is the best farming range for TH8. However, you may want to wait until you have level 5 barbarians, archers, and balloons, as well as level 3 minions before you go above 1700. Below 1700 there are tons of TH7's and above 1700 you start seeing more TH8's. About 60% TH8, 30% TH7, and 10% other. At 1800 you pretty much stop seeing TH7's and see a mixture of TH8, TH9, and the occasional TH10.