Base Cation Saturation Ratio (BCSR) is the more complex and controversial side of CEC.  To calculate the BCSR of your soil, first determine how many cations your soil can hold (the CEC), then measure what percentage of that whole is filled up by hydrogen, calcium, magnesium, potassium, and sodium.

Over the years, various scientists have hypothesized that there's a perfect ratio at which your soil achieves peak health and highest crop yields.  This idea may have begun with William Albrecht in the 1930s, and in its most recent incarnation can be found in Neal Kinsey and Charles Walters' Hands on Agronomy.  In the latter, the authors posit that soil cations should exist in the following proportions: 60-70% calcium, 10-20% magnesium, 3-5% potassium, 1% sodium, 10-15% hydrogen, and 2-4% other cations.

Others argue that what BCSR is really measuring is the far simpler concept of pH and that the increased crop yields organic practitioners notice after trying to correct their cation ratios are simply due to liming the soil.  I haven't read enough to decide what I think about this controversial topic, but I'd be curious to hear pros and cons from folks who have read about or practiced BCSR in more depth.

Meanwhile, if you're interested to see how your soil stacks up, you'll need to look on your soil test for the fields "% saturation K", "% saturation Mg", "% saturation Ca", and possibly "% acidity" and "% base saturation."  In most cases, the three first three fields can be added together to get the last field.  Subtract the saturation of potassium, magnesium, and calcium from 100% and you get the percent acidity (which is the percent hydrogen, and will be 0 if your soil has a pH at or above 7.)  Here's the data from my soil samples: