Superfissure,
You said... "... of course when you first push against the anus it's spongy anyway so not sure how far in the finger has gone."
... I can only wish that was true for me. lol. My muscle is hard as a rock, not at all spongy. It's tight and spasm-y all the time. Though I do apply the ointment internally too, it's quite a bit more time and effort for me to get the muscle to "give" enough to get a finger in. I use medical gloves to avoid any problems with application. To be economical I cut each of the fingers off, so I get four applications per glove. The surface of the glove also makes it easier because with the ointment on it, it's kind of a smoother glide than a finger. But application is really just whatever works and is most comfortable.
(You don't really have to worry about going too far in. In fact, if anything, the extra bit of dilation you're giving the muscle is probably beneficial.)