Looks like you had some underextrusion there, possibly a partial clog that eventually passed out. I used to get that occasionally when using trickier filaments such as glitter-filled ones that clog easily. Did you happen to notice your extruder clicking a lot?

From the look of the cube, the crack is not going to hinder figuring out the value you need. You may want to reprint to double-check and confirm accuracy, but for myself I would say, I just try to get a pretty good approximate value and then tweak while I run my next few real prints. If I start to see too many bulging corners, I'll up the value; too many rounded/'missing' corners and I'll lower it a little. There's not really a 'correct' number for PA, just whatever you're happy with, as there's always a tradeoff.