Thank you Thomas for the correction, see post below.
Brightness of scene, time, film rating, and aperture. Three of those four variables are needed to solve the exposure question.
So for example a sunny 16 front lit scene (EV 15), the normal setting for FP4 (ISO 125) at f/16 would be 1/125th of a second.
You are changing the aperture and need to apply the f/ number indicated. F/158 for the pinhole or f/55 for the zone plate.
To get close you can just count stops. From f/16 1-stop less light is f/22, 2-stops f/32, 3-stops f/45, 4-stops f/64, 5-stops f/90, 6-stops f/128, 7-stops f/180
The corresponding times count from 1/125 1-stop brighter is 1/64th, 2-stops 1/32, 3-stops 1/16, 4-stops 1/8, 5-stops 1/4, 6-stops 1/2, 7-stops 1
So for the pinhole (f/158) in "sunny 16" (EV 15) with FP4 (ISO 125) somewhere just longer than 1 second would be the target.
The zone plate's target time would be between a 1/16th and 1/8th.
If you move from the sunny 16-EV 15 situation to something darker, say EV 10; you will need to adjust the speed 5-stops to let in more light.
So in an EV 10 lighting situation the time would adjust from the sunny setting at 1-second to about 32-seconds (2,4,8,16,32) then factor on reciprocity.
Here's a good reference for EV numbers to estimate with. http://www.fredparker.com/ultexp1.htm