Sales tax and charges template: rate over combination of two previous amounts

ERPNext Selling/CRM
#1

I have been able to add Freight as costs in the Sales and tax template, as well as the corresponding tax.
What I would like to add is a line for “fuel costs”, a sort of fee we pay depending on the cost price of fuel.

This would mean I would like to have my final VAT on previous row amount, to refer to the sum of the previous TWO row amounts. Would this be possible?

So I have:
1 initial VAT,
2 Freight,
3 Fuel surcharge (percentage over 2)
4 VAT (percentage over 2+3)