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 too low and 39
degrees
too high when set at 250 deg F (yep, down to 100 and up to 289!) and +/- 70
degrees when set at 350.
Obviously this is crazy and totally unacceptable, but I need to know what is
acceptable for good
reliable baking. The repair people are worthless in this regard, and I'm
sick and
tired of throwing out food, as one time it burns, the next it is raw, and
sometimes it comes out with an additional hour of oven time!

I've searched the internet for weeks and found NOTHING even remotely like an
industry
standard, nor can I find a "rule of thumb" standard in any baking/cooking
site, so far.
I posted this message to rec.food.baking and they told me about
this group (much more likely to know).
Please help!

TIA
Bob in Los Angeles