-6db vs -3db speaker roll-off for sub crossover calculation?

Thanks, y’all.

P.S.-- my ears have not been reliable about setting subs in the past, and I currently lack any tech to get objective reality figures..

I know, I know. It sounds weird. Let’s call it post sub calibration trauma syndrome—PSCTS. It renders me insecure without external guidance, at least to start. 

Since I posted, I found a frequency drop off chart from a site that has allowed me to estimate where the -3 DB rolloff is, which is 12 DB higher than the -6, by the way. Anyway, I will use this to begin if no one else chimes in.

The idea of using a calibration tool is even more intimidating to me that the idea of getting subs and figuring out how to make them work.