The general rule of thumb is that if you are using 8bit,1 start,no parity
& 1 stop i.e.10 bits per character, then your timing needs to be within
10% of nominal.(allows the sample point to 'move' by 1/10th of a bit time
each bit.)
But this assumes that your inter-character delay is longer than one bit
time (this will allow the re-synching of each start bit correctly.

If you can't control the inter-character delay, then you could transmit
with 2 stop bits, this will allow the receiver to re-synch.

Alternatively get within 5% of nominal, and you'll have a bigger margin.

Phillips have an application note for the 8051 which explains off
frequency crystal effects, its number is AN448. It should be available
from www.semiconductors.philips.com