OK, think of it this way. 90% of the their intake is their regular feed you provide, 9 meals out of 10. So, if you tossed just a wee bit of every other day, you'd be OK.
Obviously, if you only have 4 birds, this would be a very, very small portion. If you have 10 birds, a slightly larger portion. This isn't complex mathematics, but just use common sense. Chickens can easily be spoiled and can easily seek those things you provide as extras and start neglecting their regular feed, in hopes you'll come by with more and more. Just use restraint and common sense. Gobs and gobs of scratch and meals worms every day? You'd be approaching a huge percentage of the feed intake.
We enjoy the chicken's reaction to such things too much. They trick us. Don't fall for it.