Remaining fermentables and resulting PSI - wild guesses?

[Cider Wraith]

Began corny keg fermentating and now considering exploring a bit of natural carbonation … don’t yet have a spunding valve and aware that's a necessity.

Four days ago started 3.75 gals off-the-shelf apple and on day two / three got the most activity, now seeing a small bubble burst every 30 seconds and wanting to experiment with a little natural carbonation

Any wild guesses as to fermentables remaining and if sealed up what PSI might climb to? Dangerously high or nah?

 

[Cider Wraith]

Back to the guess-work of bottle-conditioning. Maybe from here on often running a Spunding valve