B31.3 equivalent cycles used for "f" stress range reduction factor

[Mjconnol]

Question

If a system is subject to one displacement range from ambient to -100 F cycling 7000 times and a second displacement range from ambient to +500 cycling 2000 times how is the equivalent cycle (N) calculated for use in the specification for "f" in the displacement range case -100 to 500 allowable stress calculation.

I am having difficulty understanding the code explanation to calculate "N". Is it necessary to calculate all three displacement stress cases ie amb to -100, amb to 500 and -100 to 500.

If so then if the largest case is the range case -100 to 500 how do I specify the associated cycles in the code formula if that's what I am trying to calculate to begin with.

Any suggestions or guidance would be greatly appreciated

Thanks

Mike C