For pulls on a flat front cabinet, the bottom corner of the pull should be equidistant from both sides of the cabinet. For Shaker-style cabinets, the same rules apply as with knobs: center the pull on the vertical stile, with the bottom of the pull even with the top of the horizontal stile (or the top even with the bottom of the horizontal stile, for a lower cabinet). These aren’t hard and fast rules—you may want to adjust them depending on what looks best for your cabinets and hardware—but they’re a good place to start.
Let’s talk first about metals. As a general rule, I like to keep the different metals in a kitchen to no more than two. If you have stainless steel appliances, that counts as one metal. So that leaves one other metal you can bring into a kitchen. If you’ve got brass light fixtures, that’s your one other metal. Your new hardware should be nickel or brass.