A little confused by your post. You mean 1/4 then 1/8 and kept that for gritty mix? That sounds right. And yes the rest can be used for the 511 unless there are some really big pieces. You can take them out. I keep anything that falls through a 3/8 screen. Some people use 1/2.

Actually what I did was filter the bark through a 1/8. Basically got dust at that point to which I added to my seed starting mix. After that I screened through a 1/4 and what was filtered through is what you saw. What I used for my gritty mix were the chunks that were not filtered through. However I had those filtered through a 1/2 to make sure the pieces weren't too big. I was blending a lot lets say that. My gritty mix is posted in the other post about the Blood Orange tree and Reliance Grapes. I was told the mix looked good. I was wanting to save these "fines" for some of my other plants I planned on growing in my basement.

The pieces in the picture above look perfect for the Gritty Mix.

Josh