I plugged in my headphones and did both.
First I opened a couple of the original SH4 music files and checked the peak level.
After that I opened the file to be edited and made a approximate peak level match, forexample 20 0000. I tried to listen closely on my headphones to hear if it sounded like a similar volume level. Then I saved the file.
After I have changed volume for a couple of the new songs I went to the music directory and compared the ones I changed in the music editor program with the original files in SH4 music directory (DATA/SOUND/MUSIC).
If it sounded OK, I would start the game to check in the game itself.
If you have good ears you will be finished with the volume adjustment. If one of the songs for instance still is a bit high or low, you just have to exit the game, find the file and change it so it's perfect!
Sorry if there are some grammar errors. I'm trying to write as good english is I can. Being a member of this forum surely gives me some training!
You're following?