Experiment:
My country spal around 50.
Other country put me into spy DR doing 50 Stir Rebellion ops (17 succesfull).
Then, 3rd country drops land to 1 acre, gets spal like 200K (200k spies) and tries to bio me. He fails 17 bio in row (0 succesfull), then i sell all spies (0 spies from now) and he fails 10 more and again 0 succesfull (with 0 spal on me).

Сonclusion: country into spy DR cant be affected by spies. It seems unfair for me. Spies is nice part of game, why remove them from killing?

Suggestion: i assume probability is like (attack/(attack+defence))*DR, so, even if attack/(attack+defence)=1, we still have big chance of fail spy op. Make DR based on attack/(attack+defence) , like DR = old_DR+attack/2*(attack+defence)
This formula is first idea, so i dont offer directly this formula, just an idea.