Just re-read my post and realised I was talking rubbish, of course if you have an Estimated pull of 30 Tons then the loading at the Snatch block will be 30 Tons but the loading on the ropes on a 2:1 will be 15 Tons as will the loading on the winch, if you have a snatch block which is rated at 15 Tons then that will be the maximum load you can apply to the point were it's connected to the tackle layout. So the maximum winch pull you can apply will be 7.5 Tons.
But as I stated before, I would be very, very wary of putting any load on a snatch block were you have no history, it may be damaged, it may have been repaired after failure, you have no idea of weather it may have been overloaded during it's lifetime.
I'd suggest working out an estimated pull before any winching task, so at least you have an idea of what sort of load you are applying to your recovery tackle.