You want to extract the VAT out of a amount to be paid?

A2: brut, gross (the amount to pay)
B2: Key
C2: VAT

C2: =ROUND(A2*(1-1/(1+CHOOSE(B2+1,0,7,19)%)),2)

For B2, assign Data|Validity|Source|List|0,1,2

C2: =--DOLLAR(A2*(1-1/(1+CHOOSE(B2+1,,7,19)%))) is an alternative, using a text-currency function.

Imagine using a "speaking" key as 6.8 or 8.6, you get:

C2: =--DOLLAR(A2*(1-1/(1+B2%)))

Do not forget to change Data Validity to 0,6.8,8.6

If you use A2: net, then use: C2: =--DOLLAR(A2*B2%)