I like to leave at least half. Some is necessary for yeast health. Back in the day i used a sanitized wire mesh colander and poured my kettle contents through it. Then i started using a racking cane to transfer, which i didnt put all the way to the bottom if the kettle, so it left quite a bit of junk. Today i do a whirlpool and my kettle has a dip tube so a mound of junk is formed in the center, while the dip tube takes relatively clean wort from the edges. No doubt a good portion of junk makes it to the fermenter, but through the years all these methods have worked fine.