What is the best Line-in Source Level for Amazon Echo Dot v2?

Just to add to this, I can not get the Sonos play 5 to auto play. I have tried every single line in level with no success.

I have connected a echo dot to my connect amp via line-in.
You then have to go into settings on the sonos app to turn on the line in so the sinus will play via the audio via line in only.
You can only use prime music or Spotify if playing through the dot.
I use Spotify so just had to use the Alexa app to change music priority from amazon to Spotify, so now all my music is directed to the Sonos via echo from Spotify.
I have the volume set on the sonos at about 3/4 and the volume on the echo at number 6. So I can use Alexa to turn it up to 10 if I wanna listen louder.

Yup done all that. However, it's not autoplaying reliably. I have read that this can happen when line-in level is set too high or low (not sure which). So, I was looking at what others are using.

I had my Sonos Line In level set to 8 and the volume on Alexa set to 6 (you can see the number in the Alexa app) and it seemed to work just fine for me. I have a Connect though, not a Play:5. I'll note, however, that the autoplay did cut off the first couple of words Alexa would say. Not sure if having the "Ready for command" sound turned on would possibly prevent that, as that sound isn't particularly loud or long, so Sonos might not detect it.

I eventually disconnected Alexa from my Connect, though, as the audio delay drove me crazy.

The Echo has a setting that forces a confirmation tone when you state the wake-up word. That will wake up the line-in on Sonos. The setting on Echo is Settings > Your Echo’s Name > Sounds. From there you can turn on Wake up sound, Play a confirmation sound when you wake the Echo.

Latency is just long enough that even on uncompressed, the A/V sync is unacceptable (if you're trying to watch streaming vids where mouths are moving). I'd say it has doubled from the barely noticeable 70ms from direct line in on the player. It's fine for Youtube music, audio books, etc.