Whether we are talking takeoff or cruise, gross and net vary by prescribed gradient capability deltas. This goes some of the way to addressing differences between the idealised certification and degraded real world performance paddocks.
Planning is done with reference to net data. In anger, you might do a little better and achieve somewhere between published gross and net.
So far as ceiling is concerned, in smooth conditions you should expect to do a little better descending, rather than climbing, to ceiling