Useful Plugin!

I love this plugin 🙂 It is very useful for my projects, but now i got a short problem. May anybody can help me with that?

We want to have a calculator for selling our tables online.

for example (in cm):

Fieldname2 (a): 140 = a
Fieldname6 (b): 80 = b
Fieldanem4 = SUM

0.05 cent / cm^2

But for different sizes, whe got different pries.

at the endprice of 750, he sould get discount of 20%
at the endprce of 800, he sould get a discount of 21%

So I tried to set up the Function:

fieldname2*fieldname6*0.05

if(fieldname4 <= 750) return fieldname2*fieldname6*0.05*0.8;
if(fieldname4 <= 800) return fieldname2*fieldname6*0.05*0.79;
if(fieldname4 <= 950) return fieldname2*fieldname6*0.05*0.78;
if(fieldname4 <= 1000) return fieldname2*fieldname6*0.05*0.77;
if(fieldname4 <= 1050) return fieldname2*fieldname6*0.05*0.76;

Can anybody help me?

best wishes from austria 🙂