Trevor,

The correct tempo (and count-in) of time sigs of 6/8, 9/8, or 12/8 is a group of 3 8th notes, not a single 8th. So this is a count-in on a dotted quarter. (Note: I am referring to the usual situations, and there are exceptions since people are free to notate music in different ways*)

It helps if we talk about a song example in 6/8. Let’s talk about funiculi funicula
Here is a link to sample of the sheet music
https://www.musicnotes.com/sheetmusic/mtd.asp?ppn=MN0067310

. and the tempo is usually about 140. That’s because the 140 is the tempo of a group of 3 eighth notes.
Imagine you are hearing that song, and clapping along - you wouldn’t be clapping at 420 which would be every 8 th note. You would clap every group of 3. So the BB countin is the same thing, and is at 140. The countin is four bars of 6/8. This is usually noted on the lead sheet by a dotted quarter symbol followed by “=140”

To hear this in practice, listen to this USF marching band.specifically, listen to their countin (and watch the conductor) which is exactly the familiar BIAB style countin 1-2- 1234 at tempo about 140. https://youtu.be/5yTUB1fyQ5Y

Does the countin that USF Marching band uses seem correct?

Peter

* an atypical example of 6/8 is when it is grouped as two 8ths per quarter, as in Christina Aguilera you lost me song. I prefer to think of these as 3/4 but it gets notated as 6/8. This is the exception to the rule. https://youtu.be/WOKI_tIBWVI

Have Fun!
Peter Gannon
PG Music Inc.