The following is the link to RMA's wheat appraisal handbook. Specifically review physical pages starting at page 12 for process. Then go to tables H and I on pages 55 and 56 respectfully. Looks like ther is variability depending on type and state from .73 to .5 on tillers. 

Note:  These appraisal procedures work on low yields and will assist you in estimations.  Also these procedures are the science, the art is in taking the samples. 

http://www.rma.usda.gov/FTP/Publications/directives/25000/PDF/07_25430.pdf

Enjoy

Forgottonian