10, 15 years ago, when I helped an old neighbor do weekend side jobs here and there, I'd watch him do calculations like it was nothing... He'd explain what he was doing and I never paid much attention since I was basically there for labor. I remember him taking temps and what not then adjusting pressures like it was nothing... When ever I had a problem, I'd ask him what I might need to look for and he was always right. He was like you, and he did this for a living.
Me... I'm just a DIYer because I like to make sure things are done right and it allows me to save a few bucks. Now getting back to doing it right is why I want digital gauges. I don't want to take a chance on messing something up since I don't do this on a regular bases... These digital gauges basically tell you what you need to do... You pick the refrigerant, hook up the probes and bam! They tell you... Hard to mess up, unless the gauge is bad..... Plus, I like tools..
BTW, what I'd really like is the SMAN460, but them boogers are way to expensive ... not as bad as the iManifold, but still priced up there...