There is a formula based on the vehicles weight, trap speed, and the dreaded 'fudge number' or percentage for drive train loss all figured in to reveal the horsepower. Still a crap shoot, but an IDEA of the output.
My dyno is the width of my grin as I mash the loud pedal.
Even the big dyno shops correct their numbers for altitude! So what now? Even the parameters of a dyno cell change due to atmosphere. I think they have a 'fudge number' for that too.
I generally avoid conversations in bench racing when asked of my 'HP' and quarter mile times. It always leads to a joker in the crowd with a reason to ONE-UP you in some way, shape or form. Especially the rice crowd. You're doing yourself a dis-service by essentially deducing your hot rod to a NUMBER. You know, I know there is more to this hobby than that.
Even the ricers are hot rodders too. It's all the same, fundamentally. I remember the days when I started doing this stuff, only for me it was Novas and Chevelles and old pickups.
Speaking of Chevelles, do you think GM rated the LS6 454 accurately? Hmmm. I remember a 5000 pound sled that would MOVE.