More specifically, the Toxic limit is 1/16 of max HP, rounded down (unless rounding down would give 0, in which case round up instead), times 15. The most direct way to demonstrate this difference is to imagine a low-level Pokemon with exactly 31 max HP: taking 15/16 of that will give 29, but 1/16 rounded down is only 1 and in fact the damage increases by just 1 at a time, topping out at 15. Such a Pokemon is therefore guaranteed to be able to outrace the Toxic timer without having to use anything more complicated than Recover or similar moves, for as long as those moves have any PP left.