When using borax and cat food to kill ants how much of each do you use?

You're doing it the hard way. Boric acid is a staple of off the shelf baits- you are trying to manufacture it at home and hoping the species of ant you are after will be drawn by the cat food. Search Fluoridated ant bait stations- they are commercial quality and the best I've used yet. Irresistible to almost all ants.