Hi,
I can suggest you to download source codes of bootloader of the chip you use, and then look into file Atmelispuartbootloaderserial.a51 at function named "autobaud".

Oh well, in case you have no time I already did it for you:
- at main, the autobaud feature works "on-the-fly" and it needs with only one `U` sign to be sent. The routine inside is very simple: it just counts a time between start and stop bits by run/stop timer1 or timer2 (depends on has a chip the BRG or does not), then makes some calculation and loads BRG or RCAP2L/H with calculated values for UART bitrate.
Good days!