The 100% line is the level of your pure nicotine that you add to your VG/PG mix. The second nicotine line is where you input the level you want. For instance : If you use 100% nicotine and want to reduce it to 24mg, you input 100% ( 100mg/10% ) on the first line and 24mg on the second line.
Also : You have 5 posts on the first page asking the same question. I suggest you read up on nicotine levels and the faq that is included with the calculator. Phrase your question once and be patient for any replies.