I suggest having on field in table that holds the price excl. VAT (PriceExclVAT), and one with the VAT (VAT) value (i.e. 0,175 or 0 or whatever).
Calculated field then has formula:
=[PriceExclVAT]*(1+[VAT])
If you change VAT or Price, the calculated field will instantly update to reflect the...