Inaccuracy of a RS-232 baudrate
I don't know if there is a formal definition. But consider this:

An RS232 character is about 10 symbols long (start, 8 bits, stop ).

If you start decoding halfway through a symbol, and your clock is 5% off, after 10 symbols you will have drifted off target by 50% of a symbol width; i.e. 5% is the maximum possible tolerable error. And that assumes the other end is clocked correctly; if it had an error in the other direction, you would decode it incorrectly.

So a 2.5% error in opposite directions at each end would also be on the edge of failing.

Signal integrity problems caused by cable length will only make this worse.

So practically, an error below 5% will work with a "good" clock at the other end and a very short cable; for robust communications you want a clock error below 2%, and better than that to support long cables.

Now find out what your ATTiny is capable of, at different baud rates and CPU clocks...