Estimators' Accessories Take-Offs by McKenzie Iron and Steel, Box 3695, Eugene, OR 97403; $26.95 includes shipping for set of 3 plastic overlays. Basic Construction and Materials Takeoff by Rudard Jones; Small Homes Council, Univ. of Ill., Champaign, Ill.; 1981; $12.00. Every builder has committed at least one memorable estimating snafu, but I'll bet you've never been responsible for a blunder like this one. Several years ago a local contractor went bankrupt after getting the bid for a bunch of condominiums. As the story was told to me, it seems that only after beginning work did someone realize that each condo was a duplex, while the bid had been based on single units. It's hard to cover an error of