A 50% increase in difficulty is a 1/3 reduction of mining speed.

You have to divide the difference between difficulty A and Difficulty B by Difficulty A and then multiply that by the mining-rate in Difficulty A to get the mining rate in Difficulty B

(1.379-0.877)/0.877 *1.146 = Your answer. (0.65)

I really hope people who are throwing money at mining are capable of 5th grade math