@MoaMoney: Sadly it isn't possible to assign a different ringtone volume while charging your Galaxy S8. Although it's not ideal, I recommend raising the ringtone volume to it's highest setting before placing it on charge, by going to Settings > Sounds and Vibration > Volume > Move the Ringtone slider all the way to the right. When you take it off charge, swipe from the top of the screen to the bottom and tap 'Sound' so it changes to 'Vibrate'. Hope this helps.
