How to use percentage value in combination with the + or - value

Hi, I'm using version 1.5.6.4. So far the only but very important problem I'm running across is the "OPTION VALUES".
The products we are selling have two related prize options.
The first is the width of the product. The option of adding a value (+) for each increasing width works fine.
The second option group is the thickness of the material and this is where it gets tricky. This value would need to be in +% but the only possibilities are + or -.
Example:
Base prize for an element of 200mm width and a thickness of 5mm is 30.00€
The option value for an element of 300mm width would be 15.00€ giving a total of 45.00€ per element.
The option value for an element of 400mm width would be 30.00€ giving a total of 60.00€ per element.
So far perfect. Since this are fabricated elements, twice the material used would double the prize.
The problem is now the thickness. Base elements are 5mm, than there are 6mm - 8mm and 10mm and 19mm. But the prize of a 10mm sheet is not the double of a 5mm sheet and the others are not linear within the two extremes.
Now adding another plus value leads nowhere.
Since the difference in price for the different thicknesses depends on the Base prize + the option value of the first option, the only possibility would be adding a percentage based on the result of the first option.

Establishing a prize for each combination is not possible due to the number of combinations. Each model comes in 10 different width options and a total of 5 thickness options. This would need to establish already 50 different prizes per model and with over 100 models (rising) this would be already over 5000 different prizes at the moment.

If somebody can give me the information in which files and where I would need to implant the percentage calculated option, this would be all I need. I'm fluent enough with php to do the coding myself but I have no idea where.

Best regards
G. STRASSER

This extension may be useful: https://www.opencart.com/index.php?rout ... n_id=32506