The reason adalbert (or whoever origianlly did it) was able to decipher the effect of SD being a three digit percent was BECAUSE of tests in the first place. That is WHY we know what SD does. TESTS. 900 SD = 90% ; 899 SD = 89.9%
its not "Resto SD puts fires out haha kk tnx" its that adalbert noticed a correlation between the SD number and the amount of instant damage that you take. if you have 500 SD, which is 50.0%, adalbert noticed that you mitigate to burning 50% of the damage. He noticed further that if you reduce the SD number to 200, it had a direct effect on the instant damage you recieve. You now only mitigate to burning 20% of the damage you recieve.
EDIT: also that whatever damage you didnt mitigate IE: 200 SD means you dont mitigate 80.0% is also the exact value that the red marker drop for every unit of damage you take.
Thus SD is to give you the chance to reapir, and live beyond your original DP lifespan. Its a very clever conception, but completely fails in that new players have no clue.
Bravo to danita for taking time to quel rebellious thoughts, hopefully more new players read up on these things.