As alluded above, pepper spray is a "last ditch" defense. You already should be off your bike and be under such imminent attack that a "reasonable person" would spray. You should be off your bike not only because that act alone often suffices to give dogs pause, but more importantly it keeps you from swerving into traffic. It also improves your aim so you will have some spray left over for the next encounter.
As for people, you should spray them only right before you call 911 to have them arrested. Of course, they need to have broken a criminal law first. And (IMPORTANT) this is subject to the interpretation of the police officer way before the more retrospective part of our legal system can be applied. Fortunately, a person acting close up in an overtly physical manner with you having no alternative to retreat can be construed as criminal assault. It he/she touches you, or your clothing, or dashes something out of your hand the crime becomes criminal battery and the officer is much more likely to help you.
"Overtly physical manner" is also subject to interpretation and the "relative physical condition / situation" of the combatants is obviously considered. In rural areas the fact that your assailant or your assailant's family may be known to the officer may or may not help you. Finally, as mentioned above, even if your assailant is arrested, he/she will likely be released within 24 hours and how far away will you be? 70 miles? Only an hour's drive!
I have been attacked by dogs and found Halt! to be ineffective. Its like trying to write your name in the dirt while peeing and ending up with only "Cycl...". As an experienced bicycle tourist I want my equipment to have multiple uses so I carry Counter Assault bear spray and would not hesitate to use it in any situation where I was potentially a victim. I will just make damn sure that a "reasonable person" would also consider themselves a victim.
Nine seconds of bear spray costs $40, but a only one second blast at one foot full in the face of any dog or human assailant should suffice.