Clock prescaling setting effect on UART

Hi,
I'm trying to make some power savings with my 32U4 arduino and some problems occurred.
I have external 16Mhz oscillator
Device is working with clock_div_2 and with RF Serial.begin(19200);
Without any divider setting the device is working with Serial.begin(9600);

Now the problem occurred when I tried clock_div_4. I cannot find correct RF Serial.begin( frequency value.
I have tried to multiply 9600 with 2,3,4,8 etc but nothing seems to work.
Any idea how can I find the correct value?

Thanks,
-Arto-