How about if the potion increased your Literacy to close the gap to 100 by, say, 50% (rounded down) plus 1d6+9 (that is, 10-15)?
So if you don't have literacy, you'll get literacy of 60-65.
If you have literacy of 50, then it'll increase it by 25, and then by 10-15, bringing it up to 85-90.
If you have literacy of 25, it'll increase it by 37 (half of 75), and then by 10-15, bringing it up to 72-77.
To have it immediately kick you up to 100, you'd need at least 70 to begin with (at which point there's a 1 in 6 chance of it going up to 100). It requires Literacy at 80 to guarantee a boost to 100.
So it would take more than two potions of literacy (in theory) to get a PC's literacy from zero to 100, but nobody gets ripped off by already having literacy.