How deep to fish varies so much with time of day and where you’re fishing. If you’re up at LOTW as an example where I fish alot, you could have a constant bite all day at 30 ft. out on mud flats and at the same time, there are walleyes that may also move up into shallower water or onto points and humps at times to feed and then drop deeper again.
Every lake is the same and yet slightly different, but on most days it requires some moving around to find walleyes. Look for walleyes shallow, deep and in between if you have to. There really aren’t any shortcuts. It’s part of fishing. Use your maps and electronics as much as you can to pinpoint likely areas and talk to local baitshops for the latest info about where the walleyes are biting. They want you to catch fish and come back so their info usually as good as it gets.
Good luck.