For chum, short float all the time, unless water clarity is an issue, in which case you need to present the jig at the right depth so that they can see it. Not usually a problem during chum season. So short float all the time. You will tend to catch the fresher more aggressive fish and less likely to catch the zombies. Also reduces foul hook up and gear loss. Irrespective of water depth I tend to fish no more than 2-3 feet from jig to float. I have fished 16 inches in shallow water.
Don't bother float fishing with a jig for coho, it can be done, but it is very inefficient compared to twitching a jig, floating a, blade, bead or roe or fly fishing. Twitching is hands down the most productive method for Coho.
For steelhead, still short float to make sure you are off the bottom, but as Noahs Arc indicates you will need to be closer to the fish during the dead of winter because water temperatures mean they will not travel far of a jig. Consider the fish is likely 6 inches to 1 foot off the bottom then you want to be 2-3 feetoff the bottom, again if water visibility allows. If visibility is less than 1 foot then you will need to be a lot closer to the bottom.