You can treat pre as the gain stage and post as the master volume stage.
Meaning using the pre set to 3/4-full for max gain at modern voicing mode on this amp. Since you have asked this question, I guess you are a newbie in this, so another pointer. Your guitar volume knob should also be set high to achieve more gain and distortion. Play around to get used to relations between the knobs. This is very important for you to achieve a desired tone you are hoping to attain.
The post will be used to control the volume you want to play at, as you are in your home, naturally you don't want the neighbours to call the coppers. This is the control for overall output, will not affect your gain setting.
Learn the relations of all the above and you will progress much faster than just working on your fretboard alone. The fretboard is only 60-70% of getting good sound with feeling. Working the knobs will give you that edge over others who are just satisfied playing in one volume. If not, stick to an acoustic guitar.
