I use the guestimating software package.  Its free, well almost anyway.  I put a dollar sign associated with all the key icons using them as a unit and then count.  During this process I get interupted and then start over several times.  This cycle repeats till I can get all the way around the building or room.  Then I write it down and multiply it out.  I make a note of problems or errors on the prints that will need to be addressed (spelled change order$).  I then go to the next item on the key.  I write it up thinking of all the stuff to put at my atvantage and fax it over to them.  You can hear them having the cow over the phone later.  I double check using  total costs vs electrical costs as a percentage, or sometimes square foot costs verses what my square foot costs are.