Exactly how you do it depends on your tax laws but these formulae should
get you going. In each case I have assumed the starting data is in A1
and the VAT percentage is in B1
Calculate VAT to be added to a base price
=ROUND(A1*B1/100,2)
Calculate selling price from base price
=ROUND(A1*(100+B1)/100,2)
Calculate Base price from retail price
=ROUND(A1*100/(100+B1),2)
Depending on accounting rules you may need to use Roundup or Rounddown
to get the right figure