Waterproofing Roof Fan Flange
Because of the low-slope, I would seal all around that flange rather than just dabbing roof tar on the nail heads. You are just one plugged scupper away from a bad rainy day...
Torch-down is great if you can get a small piece to cut up for this small job. Maybe beg a piece off a roofer you know or at a roofing job in your hood.
Elastomeric patching and coating compounds work good too, use a reinforcing fabric/mesh.