I like to use the following formula
Where VolumeSFX is for game sound effects and VolumeMaster is for the master game volume.
You can also use the following for game music.
Let it also be noted, that VolumeMaster is a variable that is changeable, as is VolumeSFX or VolumeMusic. I like using a slider or (+) or (-) buttons, instead of just inputting the value.