This is one of the most common questions that we get when chatting with clients about their space. If you don't do this every day it's hard to know the exact spacing and height requirements.
Let's get to the rough outlines first. Generally speaking you should hang your pendant lights approximately between 30-40 inches above the countertop in a standard 36" countertop height. This allows for room for you to work underneath them comfortably without them being in your field of vision too much as you look at people across from you at the island.
Again, this gives general guidelines for you that you can play with. If you are an exceptionally tall person, then 42" from the countertop to the bottom of the glass shade on the pendant could be the perfect height. If you are a little on the shorter side and like the way a lower hanging pendant looks or you have shorter ceilings and want to see a little extra cord, go for 29" from the bottom of the globe to the surface of the countertop, no problem.
Tables and tabletops are a bit different. Since people will be sitting at them it requires a slightly different height requirement. Since generally people won't be standing and working at them it's more traditional to hang pendants anywhere from 28" - 36" from the bottom of the globe to the height of the tabletop.
Again, these are good rough guidelines, but do take a minute to sit at the table and look at someone across from you and see what you think about the height for you. You may again, but quite tall and something like 38" might be totally appropriate so you can see the people across from you without obstructing your view, or the converse may be true and you like the look of a pendant or dining room chandelier that comes a bit closer to the 26" mark from bottom of the globe to the top of the tabletop.
Remember first that your pendants serve you and the people that live with you. Take into consideration anything that might affect the quality of your life and consider how these pendants can best be used to increase your enjoyment and quality of life.