Thanks for the responses. My first formal training with a shotgun was in college for police science. I can't remember if I came away from the firearms classes with suggestions of mixing ammo, or if it's an equation I formulated myself. I think I prefer the slug due to the power and more precision and range it offers. However over the years I've come to have much more respect for buckshot.
One of the deputies in our county completed the firearms instuctor course through the state LE academy and told me that they recommended buckshot only, but couldn't remember why they suggest it. That really got my gears turning.
Through my reading, I think the reasoning might be that even if your target doesn't catch all the shot, they'll still get some of it while the rest doesn't penetrate much into walls at distance.
Currently, of the dept's three duty shotguns, two of them have 7 round mags and one has a 4 round mag. For years I've loaded the 7 shot mags with 4 buckshot (to be chambered first) followed by 3 slugs. I think I currently set up the 4 shot tube with half buck and half slugs.
I think I've set it up that way because they don't have side saddles on them or even butt cuffs to hold extra rounds. The 7 rounders are in racks above the front seats, so without on-the-gun storage for more rounds, I wanted the officers to have both types of options available. I've also put extra rounds in the glove compartment and in the go-bags for our rifles so they can resupply those with shotguns.
We're very soon going to put racks in between the seats for the rifles and shotguns in all the patrol cars. To make the shotties fit, we're getting folding stocks, so I think I'm going to make a push to get side saddles to put slugs into while mags will be filled with buckshot. I do like the idea of keeping an empty space in the tube too in case one needs to load a slug. I'll also have to look into the segmented slug too. I found this vid on a quick search:
http://www.youtube.com/watch?v=qaSNgNhBcUs