The answer depends on what species of wood you are using and the grade. Where I live, we use southern yellow pine and if you consult the Southern Pine Council webpage(www.southernpine.com), assuming a floor live load of 30 psf, 10 psf dead load and using a No. 3 grade, and a spacing of 16in...
I work in the Federal Governemnt and we use RS Means extensively. Means breaks estimates down like this;
Order of Magnitude Estimate +/- 20%
Square Foot or Cubic Foot Estimate +/-15%
System Estimate +/- 10%
Unit price estimate +/5%
I know there must be other conventions out...
