10 inches or so - gives enough clearance to roll a creeper around underneath the car. That makes it much easier for dropping the exhaust, etc.
I made this type of interlocking wood blocking to get it done - car was very stable. Blocks were in 2 pieces - car is resting on first set then jacked up again for insertion of 2nd set. I actually removed that top set of 2x's on the 2nd set - didn't really need all the height since the tranny was staying under the car. If tranny is being fully pulled out - car needs to be higher for it to clear...