I have just the opposite problem. I have Part B leftover!! I just started to use it to make my own resin paste. It works with either Pt A or Pt B. I have 3/4 of a gallon left!!
One YouTuber weighs her jugs when she gets them in BEFORE she even opens them. Writes down the weights and figures the difference and uses that amount as a percentage of how much LESS she will use of the heavier one! It works for her and so far, its working for me with this last gallon I got in. Hope it helps.
If anyone knows of other ways to use excess Part A or Part B let us know.