dealing in real world objects this is kind of annoying Snap to center of object

Hi, I’m new to blender but one thing right off the bat after figuring out making the grid mm is that half the object is in the - zone x y and z I would like to be able to snap the center to left corner of the object so it aligns to the grid lines in a way that I can actually measure what I’m doing based on the grid lines. furthermore one other quick question how can I numerically add points and lines where I want them let’s say 1mm from the top left corner and then on the rear the same then select the surface between those two dots and the corner and extrude by let’s say 5mm on z if I use the mouse I might end up making crooked extrusions.
Any answers would be appreciated
terramir.

G to move or E to extrude
Press the key for the axis you want to constrain it to
Type the value
G X 5 moves on the X axis 5 units

If you want to move the objects origin select where you want it to be, move the curser to that position (Shift+S / curser to selected), then set the objects origin with Shift+Ctrl+Alt+C / Origin to 3D curser (or use the Origin button in the toolshelf)