I thought I had answered on this topic, but not sure if my answer disappeared? Sorry if this ends up being a duplicate post...
Maybe do this with a Discount-type Item? (I haven't thought this through...Rustler, you have more experience with this question, so please point out if I'm missing some problem this may cause.)
Set up these Items to use on a Sales Receipt to enter sales (see the attached image as an example):
Daily Cash Receipts - for recording tax-included sales.
Tax-included Discount - for reducing tax-included sales by the sales tax amount.
Subtotal - for showing the net taxable sales amount on the Sales Receipt.
The discount percentage for Tax-included Discount is calculated this way:
1 - (1 / 1.03475) = 3.3583%
Whenever the tax range changes you will have to change this percentage. For a 4% tax rate the calculation would be:
1 - (1 / 1.04) = 3.384616%
I would set up Daily Cash Receipts and Tax-included Discount to both post to the same Sales (income) account, so the account's total will represent net taxable sales. But you might have them post to different income accounts if you prefer.
