My guess is that the roof framing is not really trusses. Engineered trusses with span of just 12 feet would not use 2x6 as the lower chord.
Loading of that framing for short periods of time, a couple of hours, is quite different from long term loading such as several months. Spanning 3 of the joists is a good idea. If you add a jackpost under the joist that is in the center of the trio on each side of the vehicle, that effectively shortens the unsupported span to a bit under 8 feet. Doing that temporarily when using the hoist should be fine.

Another thought about the 2 x 6 joist is the fact that the joist when loaded will tend to rotate/turn at the top. It is restrained from doing so at the ends. Adding bridging or solid blocking to the adjacent joists at additional points, particularly the center of the span, helps prevent this rotation and thus increases the weight it will support without failing. This rotation is called precession.

Could you sister the joist with a gluelam beam? Or even a couple of 2x12's? That should easily carry the weight assuming it is supported with appropriate posts at each sidewall. You'd want to add blocking to resist twisting. I'm sure bus_driver's suggestions will also work.

Loading a two force member mid span is never a good idea. Truss systems should only be loaded at the joints. Having said that, if you want a great site to find the answer to your question about if there is an equation, go to engineerstoolbox.com and find your way around to beam loading. Also, google "Beam tables" and the equations will be right there for you.