That 20% is something someone pulled out of their hat! makes no sense at all. Different manufacturers and different line tests as well as different mono formulations within a manufacturers line classes (tests) will break at different percentages over or even at the rated test. For instance, class lines break at or slightly under the rated test. They have to, or you will lose your line class record. Imagine catching a 65lb striper on 6lb line only to have the IGFA (International Game Fish Association) run the tests on your line for your record, and find it breaks at 8.5lbs, you will be bumped to the 10lb line class, and if a fish that size has been caught on 10lb line, you will have no record at all.
Also, the knot you tie determines the breaking point of your system. Remember, your system is only as strong as it's weakest link.
Braid breaks at a much higher test than it is rated at because it's knot strength is terrible, so they rate it at an average breaking strain for the knots people tie. See where I am going?
Do not assume that because your 30lb braid breaks at 55lbs, that you have 55lb system. You assuredly do not, or is that knot
If you search a bit online, there are some charts, I think on Salt Water Sportsman, that catalog a number of the breaking tests of the popular mono and braids. This will give you a starting point. You then need to choose your knots for the type of fishing you do, and perfect your knots till they break at their rated percent or above, not under. I typically use 100% knots, and the few knots I use that are not 100% are used in material that is WAY heavier than my weakest link, that it simply does not matter. For instance, a Perfection Loop in fluke rig. It just does not matter. Would I use this knot for the loop in a tuna rig? Not a chance!