Here is my question...
In imperial units what should I target for the depth of the base unit vs. the open shelving above? I would like there to be a "deeper" base unit than the shelving and wonder if there are any "standards" out there to keep the looks right.
Also what should the "height" be for the base unit?
When considering the depth of the base unit you need to think about what you want to put into it. If for instance you need to store A/V equipment and Cable box etc. you want to make sure you have enough depth. 20-21" works for most equipment and allows for cables etc. to fit comfortably. You also want to make sure the bottom doesn't project so far as to make it difficult to reach over to get books etc. off the top shelves.
In terms of the height of the base unit I like to start with 1/3 of the total height of the carcass/case. In this instance, lets say if the total height is 8'/96" then a height of 30-32" works. If you are planning to put a desk in front of the cabinet (office) then you could consider making the height the same as the desk.
Dividing the 10x8' space up so the bookcase (upper cabinet) is the same proportions as a golden rectangle, leaves you with a bottom cabinet at 22 5/32".
Hope that helps.