Using Glycol or Glycerine and how it affects proof

[Young distiller]

I've heard small amounts of glycol or glycerine can soften up your spirit. Was looking online for the answer, but adding either product will thicken up your spirit, how will this affect your ability to get a correct hydrometer reading?

[RobertS]

Adding glycol and glycerine will increase the density and lower the apparent proof accordingly. You can determine how much your exact addition changes the proof by redistilling, but the ttb does not require you to account for the variance so long as you do not have "solids in excess of 600 mg per 100 ml". Which means ignore the variance and accept that you will actually have a negligible bit more alcohol per volume than listed.