The above picture is an accurate representation of the stand off type situation in which a person would have the time and opportunity to utilize pepper spray. The problem is that it is a situation made up for marketing purposes.
Selection- we're not talking about the brand or model of OC to carry, but rather the selection of the tool itself. As strange as it sounds, some either carry OC themselves or provide it to others because of the inability or reluctance to carry a firearm. Others carry it as an intermediate force option in support of a firearm. When it comes to tools as part of your personal protection program, you need to consider what percentage of the time you will be able to have the tool with you. If you spend time flying, in some schools, or in government buildings, you will likely be prohibited from carrying OC. So if you were carrying it in support of a firearm, now you will have neither. If you were carrying it in place of a firearm, now you are "unarmed".
Carry- As you can see in the picture many people just carry OC clipped to their keys. In the case of women, likely in the bottom of their purse. I have found that most guys who carry OC prefer to have it in their pocket, on their belt, or clipped to the visor of their vehicle.
Deployment- As with all other tools, this is the stumbling point. I have never argued about the effectiveness of OC making someone uncomfortable, only its ability to stop a dedicated attacker who can get their hands on you. Police most often use OC in stand off situations or spray a suspect while they are being controlled by another officer. Your best bet to actually be able to spray an attacker would be to have the OC in your hand when attacked, actually not just in your hand but with your finger on the trigger since under combat stress you will likely not be able to feel the trigger. The issue is that if you are in a situation in which you feel it necessary to have your finger on the trigger of your OC, there are probably other things you should be doing with your hands instead. Another issue is that spontaneous attacks often come from the rear and flanks. In most cases in order for you to spray an attacker, you would be facing them and they facing you.
Use- So you have your OC in your hand and someone confronts you from the front and you spray them, regardless of the quality of the spray these things come into play.
Movement- situations are dynamic and one or both of you will be moving. Hitting a moving target is hard enough when you are standing still. Keep in mind that unlike every other weapon such as bullets, edged weapons, and impact weapons, OC will only have an effect if it hits the attacker in the face. In all my years as an OC instructor and police officer, I have NEVER seen anyone move while they spray someone. As far as I know, there is no training anywhere where students are taught to move as they spray. The #1 key to survival is movement. So, at the worst time in your life the tool that you are using will cause you to do the worst possible thing...stand still.
Distance- common sense tells you that the farther away the "target" is away from the nozzle the bigger the spread of the stream. Of course there is the fogger, we will get to that. Most people who carry OC will never test it to see how far it goes, so they will have no real idea of how far it will reach. Once again while you are trying to judge distance you are standing still.
Wind/Blowback- both stream and fog are effected by wind whether it is created by the natural wind around you or the wind created from dynamic movement. Once spray is released it has no master. It will work on you as well as the attacker. The first part of the OODA loop is Observe. It is hard to observe when your eyes are slammed shut with the effects of OC. Not only will this effect your ability to defend yourself, but also your ability to navigate out of the area, or even to drive. Do you ever travel with kids, or older people, or anyone with respiratory issues? Would this cause you to hesitate to spray?
So now you are probably asking OK, so what should I carry? The information above is my reasoning for not allowing my wife and daughter to carry OC or recommend it for others. First of all in keeping with the MCS training paradigm you need to use Awareness and Avoidance skills and habits. If these are exhausted, you will know you are in a situation where only Aggression will allow you to prevail.
We are all about using pens and lights as improvised impact weapons. Legitimate tools that have an obvious use. Tools that you can have in your hand without attracting any attention.
Realizing that few will seek training for various reasons, here is the least you need to know. Have something in your hand whenever possible. If someone grabs you repeatedly, strike them in the head/face/neck until they stop. Break contact and call 911 when you feel it is safe to do so. Don't trust your life to the way the wind blows.