The auditor should be looking at the retail price and the most recent purchase invoices to determine the markup. You need to make sure they use a broad enough sample to give you an accurate markup percentage. You should work hard at the audit level to get the auditor to do it the way you want. If you can’t then you can take your numbers and use those in your reconciliation conference. It is really a game of math and reconciliations.
IARC is a 50/50 proposition. These Liquor stores and C stores are not too bad to do if you generate your own numbers and can support them. I have access to the top sales tax consultants in Texas. If you have any tough issues let me know and I will reach out.