If mackerel fillet weighs 100 grams, use 1 gram of salt total to salt both sides of the fillet. It doesn’t seem like much but the fish has plenty of it’s own flavor, so don’t worry!
If you leave the fish salted longer than 30 minutes, you risk drawing out more water and making the flesh more on the firm side.
Ideally you would use a rack to broil the fish so the fat/juiced drip down a little and you can avoid pooling.