Start with the evidence
List each quantity and unit price from the agreed source. Keep discounts or other adjustments visible instead of quietly folding them into an unexplained rate. Use the stated rounding approach consistently.
Work through the example
Three sessions at USD120 give USD360; two at USD90 give USD180. Together the unadjusted lines total USD540. Checking that sum independently catches a copied total that a quick visual review can miss.
Make the next step clear
Compare the independent result with the draft and identify the exact line behind any difference. Leave an unresolved calculation as a question for review. Do not choose whichever total looks more familiar from an earlier invoice.
Keep the source context
Treat an invoice draft as a description of an agreed business event. Keep the source agreement nearby and distinguish document preparation from approval, sending and receiving payment. These are separate steps even when they happen close together.