Usually by weight. The typical estimate is about 1/4# per bird per day. LOTS of things can affect that figure - quality (and energy content) of the feed, breed of bird, whether or no they are laying, weather, etc. Its a "thumb rule" and a good starting point, not the end of the discussion.
After that, some observation can help you adjust to meet your flock needs. Feed half in the AM, half at late day for a week or so, and watch them feed. At the end of the week, if they are walking away uninterested before most of the feed is eaten, cut back a little bit. If they are ravenously gobbling it up and mobbing you for more, adda bit more. Adjust either way about 1/4# at a time for a flock that size.
Also, and I'm not good at this, you can pick up your birds and feel the keel bone, start to get a sense of when they are thin, when they are tending fat. I want to say
@aart has a really good article on how its done, illustrations, scoring, etc. Or search the web. Here's
PoultryDVM on it (not a spectacular site, but sometimes useful for the basics). and here's the
Dummies, much the same.
I don't use the process (which is why I've not developed the skill) because I butcher a couple birds a week from my flock, so I can get up inside them and poke around for a good sense of whether or not their nutritional needs are being met - not only by considering yield and subcutaneous fat, but also intra-organ fat, kidney and liver size and coloration, etc. Not a practical solution for you.
Or "free feed". Perfectly reasonable approach, unless you have CX - and you don't.