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[CML2012]

A logarithm is another way of writing an exponent.  A standard log is base 10.  So the logarithm equation is   log₁₀ (answer) = exponent.  Therefore when someone refers to a log 3 or 3 log that means 10³  (because the base is 10 and the exponent is 3) which would equal 1000 (answer).  When someone refers to a log 3 reduction that means the 3 is negative so you have 10^-3 = (1/1000).  ( log₁₀ (1/1000) = -3  because the base is 10, the exponent is -3 and the answer is 1/1000). 

How this works for CML is they take your original percentage, like 53%, and compare to where you are now, like .053%,  .053 is 1/1000 of 53 which is a 3 log reduction because if you divide 53 by .053 it equals 1/1000 which is the answer position of the equation log₁₀ (1/1000) = -3.

Now I have read that a log 5 reduction means the chromosome is undetectable, which is true if you think about log₁₀ (1/100000) = -5.  So that means if my original PCR for BCR/ABL was 53% and I make it to a log 5 reduction I am at .00053% because .00053 is 1/10000 of 53.  So basically the lab is doing the math comparison (division)  for you each time when they say you are at a log # reduction.  

   All of you should remember this from Algebra II back in your high school day     

[CallMeLucky]

That would be your personal log reduction but that is not what they are talking about when referring to treatment milestones. Milestones are based on a lab standard starting point. For international standard the base line is 100%. 3log reduction is 0.1. Anything below is MMR. If your lab is not I.S. you have to find out what the baseline is.
Assuming you were on I.S. Your 53% starting point meant you were not at the baseline yet. You were diagnosed at what would be equivalent to about a half log reduction off the baseline.