Every line on an invoice or quote has its own VAT rate: 0%, 5% or 20%, the UK rates. Choose it in the form, or tell the AI creator "add 20% VAT" and it sets the rate for you. The subtotal, VAT and total appear on the document automatically.
If you quoted a price that already includes VAT, say so ("850 including VAT") or tick the option on the line: DocMason works back to the net amount and shows the VAT separately, which is what a VAT invoice needs.
All the arithmetic - VAT, discounts, totals - is done by ordinary code in exact pence, never by the AI. The AI only records what you said; it cannot get the maths wrong.