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I would personally match levels by ear. No algorithm provides the human factor, which can distinct between dominant part of mix.. sometimes you're balancing by perceived volume of lead vocal, sometimes drums.. sometimes by volume of first verse, because it is sequenced right after previous song. It is material dependent. Of course, current broadcast standards for loudness measurement are quite elaborate and usually provide good average results, I feel, it is usually done better by ear.

Anyway, just another tip without actually buying of anything, if you really want to use helper..
You haven't tell about your platform, but if you are on Windows, there is another way, how to do that.
Foobar2000 player has built in replay-gain scanner, which uses R128 loudness detection algorithm (so basically same as Nugen, etc.).. only difference is in target level, which is -18LUFS.
You can load your songs to playlist select all of them, hit right-click for context menu and use ReplayGain/Scan per-file track gain.. After completion, new pop-up with levels table appears.


Now, you can either
- enter that values in per-clip manner into your DAW (not big deal) and then possibly adjust all clips at once (so relative level differences are preserved)

or

- update tags and apply measured gain offset using Foobar2000 converter (generating new wav files)
to do that you need to setup built-in converter (accessible again via right click menu) for output to 32bit floating point wavs (so no dithering will be necessary and you ain't loose any resolution)


in converter processing section, you'll need to enable actual gain sourced from file tags


It looks like cumbersome solution, but it works well and that setup is necessary to do only one time, because you can easily save whole converter setup to preset. So then it became only two step process - detect ReplayGain and convert to new wavs via right-click menu.

Additionally, you can enter offset for gain apply, so if you don't need -18LUFS target level, you just modify it. Eg. for -23LUFS (broadcast), you just move first slider to -5dB and save it as another preset.