You can't make 6mg ejuice if your base is already at 6mg.
Here are some calculators. Use whichever you like best.
Online (browser based)
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ELR Calc (sorry, direct link not allowed for "political" reason - link is to google search, see first result)
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LiqCal (you'll want to use the "BF+" (Base + Flavor (Advanced) calc. See language preferences at bottom of site.)
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Steam Engine
Android Apps
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LiqCalc (you'll want to use the "BF+" (Base + Flavor (Advanced) calc.)
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Eliquid Calculator - FOSS app on F-Droid (no idea how good this one is)