Sorry, I didn't write CBSubs. I've no idea what to expect as far as it's math is concerned. My guess is it's computing exactly how it should be and you need to use a different basket variable. Try the following.

[var2_payment_gross]

All the columns in _cbsubs_payment_baskets are available for you to substitute in. I don't think there's a separate column for the promotions so you probably won't be able to do the math for it as you've done for tax.