How much can an oven's temperature vary and still give good results?

If set at 350 degrees F., what is the most it should go above or below the
set temperature?
Is there actually a standard or "generally accepted range"?
Is the acceptable range a percentage of the target temperature (e.g. up or
down 5%) or is it an absolute (e.g. up or down 10 degrees F maximum?)

My oven is way out of whack, can vary as much as 150 degrees when set at 250
deg F (down to 100 and up to 289!) and +/- 70 degrees when set at 35.
Obviously this is crazy, but I need to know what is acceptable for good
baking, as the repair people are worthless in this regard, and I'm sick and
tired of throwing out food!

I've searched the internet for weeks and found NOTHING even remotely like a
standard.

Please help!

TIA
Bob in Los Angeles