How loud should your sub bass be? What is a good sub bass frequency? Here's a good, easy guideline

The level of sub bass is something I'm often asked about.

While I seldom teach numbers, this time I'll do, as this is easy and useful.
I'm using the free SPAN analyzer by Voxengo a lot, here as well.

See the pic above.
On top left, you'll see the frequency, determined by the horizontal location of your cursor.
On top right, you'll see the amplitude of that frequency, determined by vertical location of your cursor. So I'm keeping my cursor where the sub bass is peaking (see the cross above the loudest lowest peak: that's where I'm keeping my cursor).

So the sub bass is happening around 46.8 Hz, and its amplitude is -31.4 dB.
Now, I don't know what that amplitude is based on, but that doesn't matter.
On bottom left, you'll see that the song is hitting -6.6 RMS (you have to switch Metering to DBFS+3 to get an actual RMS reading).
In bass music, let's say around from -6 to -5 RMS is a decent ballpark loudness figure for your song.

The dead simple takeaway and a great "rule" based on seeing as well as mixing and mastering hundreds and hundreds of bass music songs is this:
When your song has been limited to commercial loudness, the amplitude of your sub in SPAN should be hitting around -30 on its amplitude meter (top right).
This is a ballpark figure: some songs have a bit more (usually this takes a more minimal song), some less (sometimes songs that are really busy, like my DNB often is).
Also, if your bass has a lot of harmonics above the sub, sometimes a little less feels good, and hitting that -30 line may feel a tad much, so adjust accordingly – your ears will be the final judge.

Also, an ideal frequency for your sub bass is around 45-60 Hz.
Lower than that, and some systems will fail to play it, and higher than that, and it's not so weighty anymore.
Have it hit around this range early on when designing the song/bass, and you'll avoid many problems later on.

That's it, really. If you've never done it before, do take some time looking at some trusted songs with great mixes/masters, and you will find this to be true – I can guarantee it.
So if you ever feel unsure about your sub loudness, getting it to this ballpark means you can't be far off.