No, you don't. Just use the Simplified Field Notation option in the Calculate tab of your VAT field, with something like this (in the example below VAT is 21%):
SubTotal * 0.21
Just make sure you spell the name of the SubTotal field EXACTLY right (it's case-sensitive), or it won't work. If the field has spaces in its name add a back-slash before them in the formula, so like this:
Sub\ Total * 0.21
Be sure to follow HMRC's rules for rounding tax, whatever they might be. Forms that deal with money are rarely as simple as they seem.
There are plenty of posts on the web about the rounding of floating point numbers and how best to round the values.