You will need to search out online, the exact carbon to nitrogen ratios of each item your mixing into the compost. There are web sights that have extensive lists, that show average ratios of most composting materials. Once you have the rough ratios of N to C in each item by weight or volume, you can average that out to calculate the exact ratios in your mix, adding whats necessary to create the appropriate ballenced ratio of the completed compost mix. You can also do a search online, how to average ratios, and calculating percentage ratios by weight or volume. Simply apply those equations to calculate the average C to N ratios of your entire mix, once you find the C to N ratios of each material your looking to mix in.
Thats the most precise way to do it, though most people don't care enough to research these things...lol!
I would encourage you to scavenge a more diverse mix though. Free lawn trimmings, free coffe grounds, cardboard, kitchen vegetable scraps or even shead tree leaves. If you see people you know or neighbors struggling with raking leaves, or a lawn that needs mowed or raked. Offer to help and tell them you'll haul off that debris. It's doing good deeds, and carbon in the bank, or should I say compost bin.