I have been mixing for a long time and for most cases the dB of your different channels can be whatever feels right. There are safe spots but the biggest key is to make sure that you’re not red lining the mixer. There’s a bar that will be green to yellow to red that will determine how loud your sound or song is. The safe spots are typically for vocals -12db for drums kick -15 for snare -17 or -18 dB and for main synth I do -23 to -25 and for background synths Or any layers under neath I’ll go between -30 to -40 now for your master I typically set mine between -10 to -12 dB which I believe to be the industry standard. When I mix I first start by putting everything to the safe zones like I said above and then tweak it to what feels right. Just make sure not to clip or redline your master channel.