Yes, it's always an interesting topic!
I was advised that the deeper & closer to the internal muscle, the more effective.
I don't know for sure, but I know I was able to use this method and insert my finger to under the first knuckle.
The effects of Diltiazem seemed to be really effective in reducing pain levels & spasms for me, which allowed me to go on and heal a few months later. I also used probably a little more than a pea-sized amount each time & didn't develop any other issues.