The sidetone frequency should be the same as the offset. Turn off the break in, hit the key so you hear the sidetone but don't transmit, and match it to the CW stations tone.
Before I started developing SDR software, I used to a lot of guitar tuning apps. Why? Because a lot people, even those learning to play a musical instrument, need a bit of help matching tones accurately. If you're one of those, you can use a musical strobe tuner (box or app) next to your radio. Set your side tone to a musical pitch (E5 is a common one), and then use the stroboscopic tuner to help you zero beat the CW signal.
With some SDRs, one can line up a CW signal on the spectrum or waterfall with a marker on the display to zero beat (or offset to a sparse hole if there seems to be a pile-up).