I think that most things have already been said, but here goes with my own perspective:
I spent a while with the stoma nurses discussing the location options and preferences. My main consideration was the beltline, which I did not want to be interfering with the stoma. I opted for a couple of inches below the beltline for that reason. However, what I found in practice was that, over the day, my trousers tended to drift downwards, ending up as hipsters, which rather defeated the choice of placement. At first, I resolved this by wearing braces to keep my beltline in the right place,but it still posed problems when I was bending over and the belt dragged across the stoma. Eventually, I decided that the best option was to wear one-piece overalls (what we call 'boiler-suits') which do not need a beltline at all. These work very well for me and have the added advantage of making it look as if I work for a living! They are also useful in terms of getting free access to the stoma for changing appliances without having to disrobe.
I have designed for myself, a proper suit with a waistcoat, which attaches to the trousers and thus holds them in the 'right' position. However, as I rarely have any need for such a thing, it has never been made, so I don't really know if it would work in practice!
If I had to choose a different place for my stoma, I do believe that I would have just as much trouble deciding now, as I did at the beginning.
Sorry if this is no help to you whatsoever, but it does indicate that you are not alone in struggling with making these sorts of choices.