Form - Calculation problem

See the form on: baumans.eu/order/

There are 10 rows of drop-down menu's. A calculation will only be made if you select all the drop-down menu's in one row. Try it. :-)

This question is about the drop-down menu's below "B, mm" and ""H, mm". As they're the main thing for the calculation. This calculation should calculate the price by taking steps of 200 mm. And, in the current way it calculates 200 as 200, 201 as 200, 399 as 200, 400 as 400. But, it should calculate 200 as 200, 201 as 400, 399 as 400, 400 as 400.

The problem is probably situated in: baumans.eu/order/main.js

I tried to change it myself. But, it seems, I don't understand enough from programming to solve it. Please, help! :-)