Hate to argue with someone as experienced and knowledgeable as John T, but 60 or 70 fc per sq/ft is not enough. 100 would be more like it. You have a very dark shop, and you will experience what is known as "cave effect" where the interior roof will be very dark and since it has no ceiling, you don't get any reflected light. Reflected light is good, as it fills in the gaps and provides light from different directions, evening things out.
I have a steel building with the white vinyl backed insulation inside. I used 400 watt Metal Halide lights, but these have polycarbonate reflectors, giving 20% or so uplight, which reflects off the inside of the roof (white vinyl) and it helps alot.
The lighter the floors, ceiling and walls are, the better your lighting will be, as you will get more reflected light. Dark materials absorb light.