A variable capacitor is rather rarely directly applicable in a circuit. If the tuning range is too largely or the initial capacity is not right, in nearly all cases the desired values can be reached nevertheless by 2 additional capacitors. Merely if twice xxx are displayed, than the calculaion with the entered values is impossible.
 
Frequently this is used as so called band spread in a VFO. On this webpage you can calcute the capacitors for it.

But only rarely capacitors are available in the calculated values. Change the calculated capacitors against from you used and see, which deviations with it to arise.

If you want to know, how looks the equations used into the script, then see at the mathematical background: circuit left, circuit right.