OP, If I may suggest to your common sense.
If you insist on using a slug fine, but make the slug the THIRD cartridge in the tube.
I don't know how good your reflexes are under stressful conditions and I can tell you from real life experience, a pizzed off Bear in close proximity is about as stressful as it gets, when you're the object of its attention at the time.
Make your first TWO cartridges in the tube "00" Buck.
Up close and personal, especially when you dont have time to aim you want to mangle something on that Bear to get its attention and stop it so you can finish it off with another shot.
If you need a third, you're really in trouble anyway and the shot is either point blank or ??????
Most Grizzlies will charge within ten yards or so then stop. It's usually a false charge brought about by a fear or whatever instinct and I can tell you, it takes a lot of determination on your part not to pull the trigger before it's necessary.
My standard move is put the sights in front of its path and the instant you see hair, pull the trigger. You likely won't kill it but it will certainly stop it long enough for a second shot to be placed where it will do the most good.
If you're serious about carrying a shotgun for Bear protection, be more worried about a Black. They don't give any warning an as often as not seem to come out of nowhere.
One thing you need to do is practice taking a fast shot without warning and no time to aim.
My now deceased hunting partner from almost two decades back had a good reaction trail set up on his property. There was a trail we used to take to the tree stands on the power line on the hill above and often encountered Bears along the way. Only had a couple that were aggressive. One wanted the freshly killed Deer we were packing out and the other was just curious and succumbed to a heavy dead fir branch smacking it on the snout.
If you have your shotgun slung over your shoulder, it will take most of your time to react and get a round chambered and into a position to fire it in such a manner that you're actually going to make some sort of difference in the Bear's attitude. That type of reaction takes practice, anything else is FLUFF.
My old bud and I would park ourselves along the trail with an old tire and roll it out at the least expected time. Believe it or not, the tire won a lot.
This isn't just something you do lightly. We had to be extremely careful to make sure we weren't on the receiving end of the buckshot, when rolling out that tire.
That's why we stuck with "00"Buckshot. The pattern was only about 6-8 cm across at ten yards but that's a lot bigger than a half inch (1 cm) and it makes all the differnce in the outcome of the situation.
Another thing to consider, which choke tube you're using in your shotgun. I don't know if your Benneli is cylinder bore or not.