Forget about distance in yards.
If an animal is coming at you fast from short distance you should not be worrying about things like, is he close enough, is he too far, should I shoot yet, or should I wait a bit, if I shoot now I might have time for the second barrel etc etc!!!

You need total confidence in the reliability of your double and also the cartridges loaded in the chambers.
You also need 100 percent gunfit for precise snap shooting without seeing any sights.
Then you need confidence in your ability to use that rifle and to time your shot to give you the best chance of a good solid hit in a vital area of the charging animal.
Just my 2 cents.