How do you post GST for a meal expense without a receipt as tips do not have GST applied? (When I have a receipt I use a custom combo tax 2.5% GST and 2.5% Expense)

Keep in mind without a receipt you may be challenged on the expense, but here's how I would do it: hit New -> Bill and enter as shown in the screenshot.

I am looking for suggestions of how other bookkeepers record meal expenses when there is no receipt. There is no way to know how much the tip was on the bill, you only have the final charge to the client credit card. Since eligible GST on meals is 2.5% of the meal is there a commonly used rate for no receipt.

After doing a little math, if I was to assume 15% tip on all meals with no receipt, I would come up with a customized GST of 2.16% off the total bill. If assuming a different tip percentage I would come up with a different customized GST. I am wondering if there is a common standard used in the industry for no receipts. 

Common in the industry is no receipt, no reimbursement.