Here is my problem, I'll try and keep it as simple and clear as posssible

I have 1 item that comes in 5 different weights. So what I do is create a product and in 'More Options' on the right of the screen I create 5 optional fields each with their corresponding item weights. I then tick the 'Require user to select an option' box. Simple eh ?, well not quite cos this is where the problem lies and it gets a little complicated.
I have 5 items of each weight e.g 5 x 5g, 5 x 10g, 5 x 15g, 5 x 20g and 5 x 25g. This is a total of 25 items. My inventory is set to 25 however should a customer buy 1 x 5g item the inventory goes down to 24 and so on & so forth. Trouble is should I sell 5 x 5g then my stock of that item is in fact zero but it will show on screen that I still have 20 in stock because it is taking into account the other sizes.
I cannot fathom a way round this problem. It seems to me at this stage the only solution is to create 5 different product items and set inventory to 5 and not 25.
The fact that I am dealing here with weight is irrelevant, I would have the same problem if it were say colours I was dealing with.
I do not know what the 'Enable optional field sharing' tick box does, is the solution here I wonder ????
TA