In most cases we are looking to add broccoli and cauliflower to our dogs existing (complete) diet whether that is from a can or from kibble.
This article cannot address your question about broccoli and cauliflower if you are planning on making your own homemade diet from scratch.
This is due to the fact that there are so many factors to take into account.
For the rest of us you should not give your dog more than 10% of their total food as broccoli and cauliflower combined.
I do not think you need to be too scientific about this you can roughly guess 10% by using a tablespoon or serving spoon and a little too much or too little would not harm your dog at all.