Jun-14 Cumulative Return = 959.00
May-14 Cumulative Return = 119.00

Difference = 840 BTC mined in 1 month
Total BTC mined in 1 month (3600 BTC x 30 days) = 108,000 BTC

840 BTC / 108,000 BTC = 0.007777 = 0.77 % of hashrate from May to June