Edgriv, first and foremost, welcome to the forum.
Typically, most shotshell loads will expand at around one inch per yard from the end of the muzzle.
I'd recommend you pattern you're intended use loads at distances you might expect in your HD situation. Typically, 3, 5, and 7 yards. Then at 25 yards to see your maximum spread.
Just something to be aware of - most HD or close threat situations are resolved in a few seconds and typically with few shots fired. The most important ability it to get your weapon into action and aimed at the target. Capacity is generally not the issue. However, one must be careful to maintain the balance of the gun. Every ounce you add to the muzzle changes the balance point.
Regards