Practice gets you to Carnegie Hall.
Original Gravity is measured post-boil (right before you pitch). Pre-boil Gravity is measured after you have sparged and you're ready to begin the boil. For instance, on a honey wheat ale I did, my Pre-boil Gravity was 1.039 SG (6.25 gallons of collected wort) but my OG was 1.051 SG after the boil (5 gallons when I pitched). OG will be higher than PBG due to the sugar being concentrated because of water loss during the boil.
I've used pH 52 in several of my brews, especially if I have a large percentage of dark specialty grains (they tend to lower the pH of the wort by a large degree) and, having measured the pH, it really does work, at least for me. I've heard of people having mixed results with it and others with a subjective experience that it alters the flavor of their beer. I've not noticed a difference in mine having used it versus not and perhaps it works for the water chemistry of my area better than that of others, so really it is a matter of a trial and error for you to experiment with and see if you like it or not.
There are other ways to adjust mash pH, such as with chemical additives and grain choice, but that gets into some heavy duty brew science/water chemistry and you're better off just getting the basics under your belt for now. As long as you're in the 5.0-5.4 area you'll be fine.
And as far as pH testing I just use a roll of litmus paper I got from Amazon on the cheap. Yeah (to those of you tutting while reading this), I know it's not as fancy, accurate or compensating as a $300 lab grade, electronic pH meter, but it does the job I need it to do well enough. I'm making home brew, not working for NASA.
It's nice to see you're still into brewing and I hope you having a lot of fun expanding your knowledge and skill in your new found hobby.