The Wiki page notes that the actual potato calculation is done iteratively starting with 0.2, but not ending there. The actual calculation is 20% for 1, and if successful, 20% (of 20%) for another, and so on. But if charted, you'll notice that the number approaches 25% with each increase in precision.
TLDR: crops.js line 380 should be set to 0.25 instead of 0.2.
Explanation
The Wiki page notes that the actual potato calculation is done iteratively starting with 0.2, but not ending there. The actual calculation is 20% for 1, and if successful, 20% (of 20%) for another, and so on. But if charted, you'll notice that the number approaches 25% with each increase in precision.