Starting to design out my build, looking for critique

[AndyUK]

Andy beat me to many bits of good advice

There is a reason I picked which questions to answer and which to avoid!

Washout makes a good point about the adjustment of things. I like your ballscrew placement idea on the gantry, but it doesn't leave a lot of room for height errors - the location of the top plate will be defined by the carriages, which may or may not co-inside with the height of your ballnut support. You may want to design with say, a 5mm shim under the BK/BF12, so that you can adjust the shims to get the right height. This is why the ballscrew on mine is on the back of the L shape, it allows me to adjust for height relative to the carriages - but leads to a slightly more flimsy 3-sides of a box shape.