5 zulrah scales per dose of antidote is what's used, but it'll only ever act on that specific dose count of potion, so 20 scales per 4 dose and with 27 4 dose that's just 27.
I don't have the herblore lvl to try antivenoms, but I found a similar issue for stamina pots. I fixed it for both. Let me know if you still have the issue, otherwise please close this.
5 zulrah scales per dose of antidote is what's used, but it'll only ever act on that specific dose count of potion, so 20 scales per 4 dose and with 27 4 dose that's just 27.