If you use the stringmeter to measure adjacent cross strings and compare the results to one another it is reasonable to assume that the tension will not vary tremendously. However, the two cross strings where you start the 50/50 method will measure considerably lower than adjacent cross strings. This signifies that tension is being lost in the starting process when using 50/50 and guess where it is being lost? Right in the most important area of the racquet.
With all do respect to Irvin, please let us know where you have found any documentation indicating the stringmeter was designed exclusively for use on mains and not crosses? I'd like to read more about it.