Table rate not calculating on tax inclusive total

I think I might have found an assumption in the table rate postage in Marketpress. My store is set to display tax inclusive prices. At checkout the contents of the cart displays the tax breakdown for the order in the cart.
I have the store set for free postage for all orders over $79.99 ($80).
When it comes to reviewing the contents of the cart before checkout. With a tax inclusive total of $80 in the cart (subtotal of $73.45 and $6.55 in taxes) the cart applies our $8 shipping charge, when the charge should be $0 according to our table rules.
I do not expect this behaviour as I would expect the table rate to calculate the ‘total’ (i.e. total spend) to calculate postage rather than the subtotal (excluding tax).
If this is not reviewed and seen as a support fix, can I recommend an urgent enhancement to the product which allows the person configuring whether the total is tax inclusive or tax exclusive.
Looking forward to your thoughts on the matter.