oh my... a 1.5MB BMP :biggrin:
Don't you have a scanner? or digi cam? It's much easier to draw this with a pencil and please save it as a JPG. I think even Paint can save JPGs. Don't get carried away with exact units ect. Don't even use a ruler.
I think a strict, flat overview is the easiest and most helpful. But to give you an example for what is theoretically possible, look at
this sketch for HL1. You don't have to draw everything in perspective yourself. But feel free to make a few bigger layout sketches, then scratch and combine parts till it is densely filled with interesting stuff.
Make sure a room adds something besides being a room, for example. Don't draw a "room" draw a rectangle that you label with "kitchen" or "tire store". Make sure it makes sense to be there, also.
Think of how a base like this would be structured in the real world, which parts would be interesting and then play architect and draw a sketch for a blueprint.
Why snark works.