If you can do simple math, e-juice calculators aren't necessary for simple recipes.
Want to make 10mL of juice with 20% VG?
Multiply 10mL batch quantity x .2 to get 2mL of VG.
Want to make 18mg juice from 36mg nic liquid?
Divide 18 by 36 to get 0.5, multiply 10mL total batch quantity by 0.5 and you get 5ml.
Want to use 15% total flavoring?
Multiply batch quantity of 10mL x .15, result is 1.5. You will use 1.5mL of flavoring.
Okay, now add all those together:
2.0mL VG
5.0mL nic liquid
1.5mL flavoring
= 8.5mL total volume so far
Subtract 8.5mL from 10mL batch quantity and you get 1.5mL, the quantity of plain PG needed to finish the recipe.
I do the calculations, write down the ingredient volumes and percentages and then add them up to check for accuracy.