The measure that works

Compare against your own record instead. What could you do in September that you cannot do now, or could not do then and can now? This is the only comparison where you have both sides of the data, which is what makes it the only one that reliably tells you something.

It has to be written down to work, because memory adjusts. A record of what you could actually do six months ago is unglamorous and dull to keep, and it is the single most effective thing available against this — for the plain reason that it is the only measurement in the whole business that is actually accurate.