pls clarify what timing are you talking about .. and is this related to my earlier thread RS485 software
There is - for some reason two adjecent threads discussing RS-485, thus this thread.

when the interrupt happens and you detect that you have transmitted the last byte, you want to change the transceiver to receive. If you do that without waiting 1/2 - 7/10 bit times you lose the last half of the stop bit and the receiving end may not detect the bit.

Another thing to be careful about is that some derivatives are double buffered and interrupt when one buffer is empty. For such derivatives you must wait 1 byte time + 7/10 bit time.

Erik