How do I find Bottom Dead center?

[chris91LX]

I got my leak down tester and the instructions say to warm up the car, remove the spark plugs, set the piston at Bottom Dead Center, loosen the rockers to make sure both valves are closed and hook up the gauge. Do I guesstimate BDC and let the air pressure shove the piston the rest of the way down or what? I thought I was just supposed to set the piston at TDC for this test, ain't it funny how nothing is ever as easy as it is supposed to be? Thanks for any info.