Help needed designing square wave osc (555)

Hi, I've been trying to calculate "standard" values that would make me
a 555 R1/R2/C1 square wave oscillator with a 50% duty cycle operating
at close to 30Hz.

Can someone assist? My numbers come out with "odd" component values.

I need a 30Hz and 60Hz oscillator eventually.
Ideally a variable oscillator that could be set to 30 or 60 would be
ideal.

Thanks,
Dave