A common question we get around the shop when customers see a 800mm bar on a stock build is "do you recommend running these at 800mm?" The answer to that, in most cases, is no. Manufacturers are typically sending these out at such a wide width, so that you can easily tailor them to the rider without having to swap the handlebar out for a wider one. It's like a haircut - you can always take a little off, but you can't ever put it back on.
When it comes to figuring out what width works for you, taking a measuring tape and measuring your shoulder width can be a quick calculator. However, I believe riding style can play a huge part. Again, that's why these handlebars come so wide to begin with. As you ride, you can start chopping off 5-10mm here and there until everything just clicks. Some riders may prefer something slightly wider than their shoulder width and some slightly narrower. Once you hone in on your desired width you can carry that with you from bike to bike as you would your saddle height. Like most things with mountain bikes, it's just personal preference.