I'd say "stack" is the wrong term for the Miss Chance.
Example... Linda knows she is going to have fun with some random guy in town. Before she leaves, she applies olive oil, inserts a gnomish cervical cap and vaginal sponge soaked in honey and crocodile dung. To be really safe, she drinks an herbal tea she got from the hedge-witch two doors down.
Her partner, worried about disease, wears a sheath made from a fish-bladder.
After he is done, the pregancy roll is made... It indicates that conception occurs, but the various forms of contraception get a say now. Linda and her partner roll for the olive oil, the cervical cap, the sponge, the tea, and the condom.
If any miss would indicate that, the contraceptive prevented conception. No need to make any more rolls after that one. On a "hit", keep rolling and hope that the following contraceptives prevent conception. If only one contraceptive was used, that would mean that there's a baby on the way. This system rewards people who don't just rely on one form of contraception.
The only difficult thing here is how to deal with other forms of birth control. Since we don't have any plans for rules for menstruation, any timed birth control method would be hard to replicate with a miss chance. Apart from the true lunar cycle counting method, most of those are modern though (taking temperature every morning, etc.), so it shouldn't be a big deal.
Example... Linda knows she is going to have fun with some random guy in town. Before she leaves, she applies olive oil, inserts a gnomish cervical cap and vaginal sponge soaked in honey and crocodile dung. To be really safe, she drinks an herbal tea she got from the hedge-witch two doors down.
Her partner, worried about disease, wears a sheath made from a fish-bladder.
After he is done, the pregancy roll is made... It indicates that conception occurs, but the various forms of contraception get a say now. Linda and her partner roll for the olive oil, the cervical cap, the sponge, the tea, and the condom.
If any miss would indicate that, the contraceptive prevented conception. No need to make any more rolls after that one. On a "hit", keep rolling and hope that the following contraceptives prevent conception. If only one contraceptive was used, that would mean that there's a baby on the way. This system rewards people who don't just rely on one form of contraception.
The only difficult thing here is how to deal with other forms of birth control. Since we don't have any plans for rules for menstruation, any timed birth control method would be hard to replicate with a miss chance. Apart from the true lunar cycle counting method, most of those are modern though (taking temperature every morning, etc.), so it shouldn't be a big deal.