[FujiSkunk]

You can always ask others to test your vocals prior to release. Beyond that, if you don’t already have multitracks, audio separators like Spleeter and demucs can help isolate the vocals so they are easier to chart. While the isolation is never perfect, demucs in particular can give some pretty spectacular resuls, depending on the source material. If you don’t want to install demucs or other such software on your computer, there are web services where you can upload your song for processing, like this one.

If you use REAPER to chart, use ReaSynth or a similar plug-in to play the MIDI notes you chart, so you can compare them to the pitches of the original vocals. The C3 REAPER template is already configured with ReaSynth on the vocal tracks, making this even easier. Finally, make sure you actually are charting, and following the documentation, not just copying notes from some other MIDI source without any changes. While vocals are by far the most challenging part of authoring a custom, they also offer the most leeway, and even a small amount of effort can be the difference between a bad vocal chart and a decent one.