i charge 1.2% but what is the recommended interest rate to charge can you explain everything about the best interest rates and the best forumla to calculate it 
I don't see why btc loan interest rates would be any different than USD. 1.2% seems very low but it all depends on the amount of the loan, length of time, and how well you can verify that the borrower has stable income and means of paying you back.
I am guessing that you can't really verify their credit score or income for these loans, although I could be wrong. If this is the case, I think an interest rate of 7-10% would be more appropriate if it is a loan for like a couple years. If you can verify income and a good credit score, and it is a relatively short loan then maybe something more like 4-5% might be more appropriate.