The number relates to the degree of hardness. The harder the rubber, the heavier the BB that they are designed to lift. For .20 you want 50 degrees.
As said above, unless your site says .20 only, most regular players use .25 or .28 in an aeg, along with a 50 or 60 degree ML.
A change of 5mm either way is meaningless performance wise, it might matter aesthetically mind.