The amount of conductor heating at any given time is proportional to the current squared ( i.e, I²R ).

The effective current that topgone mentioned is the amount of continuous current (i.e., at a 100% duty-cycle) that would produce the same amount of heating as the actual current that has a duty cycle D%.

D% = 50% means the welder is on for 1/2 of the time during the standard 10 minute period.

Therefore to get an equivalent amount of heating:

(effective current)² x 100% = (actual current)² x duty-cycle D%

After dividing by 100% and taking the square root on both sides of the equation:

effective current = actual current x √( duty-cycle D% / 100%)

And so the multiplier to use for calculating effective current is: √( duty-cycle D% / 100%)

And so as topgone mentioned you get the numbers in the table such as:

√( 40% / 100%) = 0.63

√( 25% / 100%) = 0.5

...etc.