I just read your response and also some of the previous replies made while I was typing.
I like the idea of proportionality. I think it overly harsh to fail someone due to spelling alone (assuming their grasp of the scientific principles is sound).
But IMHO, since the responsibility for this decision is basically yours, I would suggest putting them on notice that spelling does count EVEN on in class exams.
You can then decide upon exactly how much it counts. Even tho I was teaching on a different level, I based my grades upon effort as well as just pure numbers.
If I had a child with clear cut issues such as dyslexia or similar but was trying as hard as he could, I acknowledged the effort.
Likewise, if I had a really really smart kid who was “coasting” and being lazy or careless, I took any opportunity to hold them accountable.
I don’t know whether the volume of your classes or workload allows for as much differentiation as I had at a lower level, one room class. But I think there is an essential difference between a foreign student trying their hardest and a lazy privileged American kid who can’t be bothered to make the effort to pay any attention to spelling at all ( and any permutation between those two extremes).
But, my basic point would be that spelling should count. It shouldn’t just be a free-for-all just because it’s an in class test. Let them study how to properly spell the pertinent subject terminology as well as everything else. It won’t kill them.
You can decide how much it counts on a case by case basis if you wish. Or you can develop a rough percentage formula for determination if that suits your philosophy better.
But please make it count for SOMETHING before our entire educational system goes to Hell in a handbasket :)