AFMwhy we use futures price to calculate gain from Interest rate Options?

Can anyone tell me why we use futures price to calculate gain from Interest rate Options ? For ex: We got a Put Option Strike Price: 96 (4%) Market rate at the time of borrowing: 7% (93) Say, futures rate at the time of borrowing = 93.5 (6.5%) Here, instead of calculating Gain from Option by 7% - 4% = 3 % we actually calculate it by (futures price) 6.5% - 4% = 2.5% Why are we doing this?