Sure, it's all possible. And you don't need a script to do it, either.
Just use the built-in options under the Calculate tab. You can use the first option to create a sum of fields and the "Simple Field Notation" option to calculate the tax.
The formula to do that that would be:
sub-totaal * 0.21
You can also create a hidden text field with the tax percentage rate and then multiply the two fields.
Thanks for your quick answer! I tried what you said. At first I couldn't believe I didn't see that for myself, lol! But it does not quite work however. This is what I get:
The second line of data in the Bedrag column is added behind the first. When I add a third line it says NaN. Also the calculations in the BTW section isn't working as well. I think Totaal works properly but I dont know since BTW is 0. I did as you said, what am I doing wrong? Thanks once again
You can't use multi-line text fields for this (well, you can, but it's much more complicated). Each row needs to be an independent field.
Hmmm that explains a lot. Thank you, you've made my day !