How to use custom field in the right way?

[MvD010]

I am making a website for a solar panel shop. I have some real problems to set the custom fields right. Hopefully somebody can help me out. I will try to explain why it's difficult:

I would like to have customers make two choices:

* The first choice is related to the amount of rows, every row start with the amount of $40
* The second choice is related to the amount of panels, each panel had a price from $25
* The third choice is to choose between black and grey (but this costs no extra money)

So for instance if somebody choose 3 rows from 7 panels is 3 times $40 + 21 times $25 = $570

If i make the choices with custom fields they cannot use the add to cart button, because if the add more products they will automaticly muliply everything, so this will be 3 times $570

Who can help me with this problem please?

[PRO]

maybe this plugin is good for you?
http://extensions.virtuemart.net/products/custom-size-detail