You should leave the conversion % increase at 0 because if the mining equipment isn't going to ROI in BTC terms then what is the point of going through the trouble of mining when you could have just bought BTC.
That is what I'm trying to figure out... if you can get ROI. If you're using a calculator that calculates your return over several months, I would assume you need to include the change in BTC currency value over that period.
For instance, if I spend $6k, then spend 6 months mining, those coins earned in 6 months aren't still worth the $213 (or whatever current value is today)... if they're worth more, that could help your ROI.