Calculations take more time than anyone wants to take in the field. Use one of the methods above and make a chart for each of the lenses you own of the exposure factor and stops to open at several different extensions (intermiediate values are easy to estimate). This you can then carry with you along with a small, lightweight tape measure. Determining the factor is then a simple matter of quickly measuring the total bellows extension and consulting the chart. I carry a small notebook with me anyway for an exposure record and bellows extension factors and reciprocity info, along with other stuff I need are there as well. Some people tape the tables to their cameras. I find the tables faster than the disc and measuring the ground glass methods.
Hope this helps a bit. ;^D)