How do I set the number of products shown using a percentage?

Hi! I would like to have the number of products in a row use a percentage of the screen, instead of telling it how many (it's set to 4 now). I like the size the products are right now, and alternately the size they are when I chose 3 per row. But, now there's a ton of space on the right side of my website, and I'd like to have that side menu hug the right side and maybe determine the number of products shown by granting the products area 75% of the width of the total area. If this is not possible, I'd rather keep the size of the products the same as it is than loose that. Thanks for helping!