Opencart Shipping method input field: I want to use Taxed sub-total

I use Parcel Pro shipping method in my opencart 2.1.0.2 shops. For every shipping option in Parcel Pro I can use 2 activation dependencies, sub-total and total weight. The sub-total field use the non-taxed sub-total. We use different tax values in our shop, so if we want to give free shipping above €50 we have a problem if a customer buys items with different tax values. The sub-total field must use the Taxed sub-total to set the activation dependencie right. Must be a small issue I think to point to another field.