IExpense match to actual tax amounts

Hi, we are looking at a new tax setup to facilitate an automatic calculation of tax using tax rules, so that iexpense tax calculation is Actual tax paid for an expenses.  I'm wondering how to address expenses such as Meals or Airfare where the tax is not based on the total amount of the expense less tax. (for example - Meal of $50 plus tax of $2.50 (GST) plus a tip of $10, total expense of $62.50. if the total expense of 62.50 is used for calculating the tax, we'd end up with a larger amount of tax then the