Most people use 1/4 from what I see. I didnt buy any milled or chopped glass for mine. I used a cross cut paper shredder and shredded the cut offs of csm and mixed it in. They came out different sizes but when mixed in worked out good. If any pieces came out to big I used scissors to cut those clumps up.
For my engine mounts I used 4x4 Douglas Fir.
Yes. I did that to help with positioning everything being my first time at this. I am glad I did this because my boat is narrower being a 4 cyl. which left me having to make many angled cuts underneath which would have made it hard to get them in the proper orientation to each other.
Not sure about wood flour, I know people use it to thicken and it supposedly has better sanding properties. I have also read others say it will absorb and hold water. $20 from us composites will get you more cabosil then you will want (i have a bunch left over and not much more PB to make.)
A good rule of thumb is to replace at least what you took out. More is ok but you will start adding weight to the boat.
