EDIT: Sorry, but if he's already spent 6 years perfecting an NES tracker, why would he spend more of his free time building us a GameBoy tracker that he would spend years developing when there's already LSDJ, and another one whose name escapes me.
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There's probably a better scale to use (one that would allow you to use 0/4/8/C/F instead of 0/2/4/6/8 for volumes)...
I'm a noob compared to you but with an FDS volume of 20, the volume column levels I use are F/8/4/0 as the GB has only 4 volume settings for the wave channel.
EXAMPLE TIME!!!
C-300 F 431
B-200 8 - - -
A-200 4 - - -
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Oh dear god, GB/GBA has horrible sound hardware limitations. You can't change square wave volume without using one of the preset envelopes, resetting the wave, or using extremely precise timed code.