Your snowboards inserts define the maximum and minimum stance width and the offset stance position, but then you can adjust them while mounting the bindings, you can also mess it up badly if you're a beginner and/or totally clueless of what you're doing and end up with board with a ridiculously large or narrow stance or more tail than nose (from the bindings out) or with 0 0 binding angles or with a negative angle in the front foot... The point is, the stance width, the stance position (and also the stance angles or bindings angles) have a major influence in your snowboarding and on how much fun you take from it.
Stance width (bindings width) How to choose your stance width? 1 Just make your feet as wide apart as your shoulders and do a few squats and adjust your feet positions (helpful for the stance angles too) as you squat, until ending up with a comfortable stance width and stance angles. 2 use your forearm, measure the distance from tip of your fingers to your elbow, that's a bio-mechanically correct stance and then you can adjust as you feel the need, making it slightly smaller or wider until you find one that feels natural to you.
Stance position (bindings position) Shall you use your snowboards offset stance position or set the stance back? There are no tricky ways to find out because the stance position only has to do with your riding style and the snow conditions. Keeping the offset stance is keeping the snowboard as twin as possible and using a setback stance is making the snowboard more directional. You can also set your stance forward to make a directional snowboard a bit more twin for a park day or something, as long as you don't end up with more nose than tail on your snowboard you're technically well set. To ride powder it definitely helps to set back your stance even if you usually go for a more twin stance and freestyle type of snowboarding.
Learning about twin and directional snowboard shapes and about snowboard types will help you on this decision. It get's down to how twin or how directional you want your board to be. Still lost? Just start with the offset setback. Mount the bindings centered on the inserts and try it, then you may try to setback your stance to the maximum (be careful here to keep the same stance width) so that you feel the difference, then go back to the centered stance and set it back 1cm at a time, keep trying until you know for sure what you want, and then keep on playing with it from time to time just to refresh the mental images.
Stance angles (binding angles) What stance/binding angles to use on your snowboard? Like the stance position, the stance angles depend on your riding style. More directional angles for directional / all-mountain riding and more twin angles to ride in both directions, for a more freestyle type of snowboarding. Twin stance angles between 9-9 to 18-18 are OK. Directional angles have a lot more possible combinations, from slightly directional like +18-12 to radically directional with both feet pointing in the same direction like +30+30, some are good, others not so much, but at the very least, make sure your front foot does not point backwards. (more on this soon)