Doesn’t UE opacity work like black = translucent and white = opaque, I can see your glass is totaly black in that map anything in grey range will be more visible. Try white and multiply with a constant of .5 (etc) to get the right look would be my process but i am a mere FX artist, Also you might wish to try some SSS instead of simple translucent. There might be some examples of water and/or glass materials in your engine or in samples i would take those and look at how Epic has them set up and trying to replicate them by matching your needs. That is the best approach i can think of.