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You're comparing our OP here, who whipped up an excel spreadsheet to do the math on his valve lash, to a dude who slapped a head on top of rusty cylinder walls with zero cleanup. There's no comparison here. Yes, some folks just need to take it to someone who knows what they're doing.
Other folks can do as good or better as a hobbyist than many professionals do. Are those guys outnumbered by dumbasses? You bet.
Are there still a lot of those really skilled & motivated hobbyists that will do a surprisingly good job simply because they're emotionally invested in THAT project, e.g. THEY GIVE A FUCK? Also yes.
The guy you're talking down to is a retired powertrain R&D engineer and project manager that worked at at least two different major automakers. I promise he knows what he's talking about, and I promise that whatever experience or knowledge you've gained by working in a machine shop pales in comparison to his collective experience with exactly these sorts of things.
Another fun fact - regular earf people with a ~$750 Chinese benchtop lathe with a decent set of carbine turning bits can do a surprisingly decent 3-angle or even 5-angle valve jobs with undercuts and the whole nine yards. It just takes a lot longer and some patience since you're carefully measuring each cut and doing them all one at a time.
Yet more fun facts - Hand-lapping valves with valve compound inside the head itself cleans up both the valve face and the valve seat. Don't even need a lathe for that, it just works. Since doing that does remove material that changes valve lash, doing that first before setting valve lash or changing shims etc is well advised.
Will a head with valves and valve seats done by hand like that flow the same as a top-shelf 5 angle job with an undercut? Nope. Will they hold compression and be very equal across all cylinders and run just fine? You bet.
Sophomoric tendencies when you're genuinely trying to help is just fine, and we welcome that. Just be willing to accept correction and understand that you're far from the only person on SF that knows what they're talking about. I am sure there's knowledge you can share that's 100% in your wheelhouse that would benefit a lot of folks on SF. But please make some effort to understand the difference between what you know, and what you think you know.
I was young and sharp and convinced I knew everything once - so I understand where you're coming from a bit more keenly than you might believe. Better to be humble than too proud.
