Need light - go for 50 ft. candles
Jim ... 50 ft. candles (measured at a height of 2.5 ft.) is a good lighting level for shop work. Some delicate tasks (such as soldering small circuit boards) are better at 100 ft candles, but this can be done with additional task lighting.
An even lighting level with minimum shadows is best, and this usually means lots of separate fixtures.
Most folks have way too few lights.
You can't use watts per sq. ft. without taking the fixture type, light distribution pattern and construction into account.
So..... I dusted off an old lighting calculator CD and did your garage.
Assuming a light colored ceiling, 4ft., 2 lamp fluorescent fixtures mounted at 8 ft high, and average reflectance walls, and a typical concrete floor.
The answer was 12 fixtures in 3 columns (fixtures are vertical in a plan view)
This gave a peak of 56 ft. candles, an average of 52 ft. candles, and only a drop off to 35 ft. candles in the extreme corners.
If you want me to change my assumptions, let me know and I'll re-run it.
gil in Tucson ... did my hangar lighting this way. Came out so well I did three neighbours too...
PS Just did Thomas' garage using the same assumptions, but only 6 fixtures. With optimum fixture spacing, it came out at 29 ft. candles peak (right under the middle two fixtures) and dropped off to 19 ft. candles in the corners, and then dropped to 18 ft candles half way along the wall between the two columns of fixtures. The average was 23.5 ft. candles.
PPS Just re-calculated Jims with a non-reflective ceiling (peaked, unfinished garage with no wall board on the ceiling), and the same lamp layout dropped to and average of 43 ft. candles, a peak of 50, and 30 in the corners.
This shows a white painted ceiling is needed to keep all of the light levels up.
The fixture I used in all of the calculations was a Lithonia "C 2 40 GENERAL PURPOSE CHANNEL, 4' 2 LAMP T12RS", which I think is the 2 lamp, open surface mounted strip fixture sold at Home Depot.