John,
As you start to use this and see how the amount of light fluctuates while the time stays the same, you may want to search the posts for help. RH Designs StopClock Vario is one timer that has a sensor mounted under the light. The sensor measures the actual amount of light put out allowing you to increase or decrease as needed. Straight time measurement without compensation for this fluctuation will drive you batty.
I am using an Aristo 12x12 head on a Durst 138S converted to 8x10 with a RH Designs StopClock Vario if I can help.
John Powers