And then in a week it drifts into a different value and you have to calibrate it again.

And that's assuming your room temperature is controlled for all experiments.

And that's also assuming the current going through it is 0 so it is always exactly at room temperature.

Virgin theoretical physicist vs Chad experimental physicist goes brrr

Still, it's useless to try to get a board of the right size by nailing together boards you've cut wrong. You throw it out and try to make a better cut on the next one.

The standard resistor values are a bit weird at first but when you’re in the zone and you start getting used to what they usually are you start becoming relatively decent at making an educated guess for what the values should be. The actual IRL values are predictably defined and you start remembering the possible options over time.

Someone’s probably made a program that can do it for you though. My own shitty circuits are all built from standard example circuits so I haven’t had to think about this stuff often. Or like I’ll mix and match them illogically in a pinch (on the breadboard) and figure it out later (I don’t build the final circuit lol). Or of course, the venerable using-a-potentiometer-exactly-where-you-shouldn’t technique, which is one of the pillars of modern engineering.