This tool helps calculate values for C & R for a CMOS 4047 in free running (astable mode). The beauty of this device is that it gives almost 50% duty cycle without any difficult calculations (unlike the 555).
Download the datasheet here
Example: Using the above formula triangle and assuming we have a 50% duty cycle ie T(on)=T(off):
For a T(on)(=T(off)) period of 0.1s then F=1/0.1=10Hz
For a F of 50Hz then T(on)(=T(off))=1/50=0.02s
Unfortunately I can only seem to get this webpage to run in IE - Sorry to all the FireFox users.