I'd start as close to the terrarium as you can get, set up the fan, and watch the temp/humidity inside the terrarium. Adjust the fan speed and light height as necessary, keeping in mind that the ballasts (the part that gets hot) will 90% of the time be on the same side of the shop light fixture as the power cord.
Ideally, you want to keep the lights as close to the terrarium as possible so your plants get the most light. Worst case, if you're not putting your soil mix directly into the terrarium (e.g. you're using the tray method with the plant in a pot of its own), you can raise the plant up on a pedestal of some kind (upside down pot, ziploc plastic container, etc.) to get it closer to the light.
Always measure temp and humidity at the plant level (put the gauge between the top of the plant and its soil).