The 12/8 shuffle is the low hanging fruit in this competition.
After watching this video, I realized that when learning a shuffle - I never had to think about it ,kinda came naturally. Probably because I did listen to a bucketload of blues before ever trying to play a shuffle.
Whats weird was I only really had to think about when I jammed with a drummer (he was learning his craft), and he couldn't play his 4/4 beats over bluesy kak I was jamming. So we had to stop and figure out why, AFter a little tinkering and playing around with a few drum loops - was a shuffle feel which he picked up super quick and all was well.
Our secret sauce was "La Grange" (still is!), to lock into something that we could both play around with the shuffle 'percentage'.
So all that to say - for me, it's a feel thing rather than X% of shuffle swing. If I'm ever lost, I just aim at emphasizing the "and" in the 1 and 2 and 3 and 4 and count.
And playing over a shuffle drum loops can help loads!