In my opinion you are best using the loudness meter in Cubase. It’s exactly suited to your purpose. A loudness meter gives you a much better idea of loudness than any other meter. It’s way more sophisticated than a digital peak or VU meter. You’d need to use the ‘Integrated’ value (in the lower half of the meter).
The integrated value is calibrated to -23LUFS (the EBU recommended level for broadcast) but you can ignore this when comparing the loudness of your songs.
I’d strongly recommend that you don’t compare just certain passages. I’d recommend that you compare ‘whole’ songs and measure the ‘Integrated’ value for each of them. Reset the meter and just play the whole song from start to finish each time. The loudness meter has gating to compensate for quiet passages. You may run into problems when comparing songs of a very different overall character… but if all your songs have the same character and use the same instruments in the same style then it should be OK.
If each of your songs is available in a single audio event you can use Audio / Statistics to find out the integrated level, shown in the lower EBU R128 section of the Statistics window. This is faster.
Depending on what your finished product is going to be you could try aiming for an Integrated level of around -14LUFS for each song with a true peak reading of -1dBTP (rough guide and depends entirely on what kind of music, how it will be distributed and so on). You could allow the final measured integrated levels to vary by around 1 or 2LU. If you want to control true peaks, use Steinberg’s Brickwall Limiter last in the chain.
For mastering purposes I’d recommend you set the loudness meter to the +18 scale in the settings. This lets you see levels higher up the scale. You might also want to activate ‘Reset on start’.
P.S. If you are using Ozone, as far as I know you can also switch its metering to ‘integrated’ loudness meters (LUFS(i) + LRA + TP) somewhere in the settings.
For more about loudness metering read this: