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

Well then you really should get something.  Room EQ Wizard with a calibrated mic would work.

The answer, in a real room is :  It depends! Your main speakers may put out a lot more or a lot less bass than you think.  Speaker specifications are usually "quasi anechoic" - meaning they pretend to be in a room with no boundaries or reflections and no room modes. I wrote about this problem here:

https://speakermakersjourney.blogspot.com/2021/12/room-speakers-eq.html

You don't trust your ears?

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. 

Not gonna matter. Your ears will work fine with the proper setup technic.

Try Jim Smith: "Get Better Sound" and have a sub like SVS that has a remote app.

Soundoctor.com is another good test cd for setting up sub(s).