Since your bowels are cleaned out pretty well, I would take a smaller dose of Epsom Salts at night, like you suggested. I only take 1 tsp of ES in each glass of apple juice (or water, or grapefruit juice - whatever) at night. THEN in the morning I take 1 tbsp of ES in a glass of AJ...and wait...in two hours if I don't feel sufficiently cleaned out I take the 2nd dose...maybe not even a full tbsp of ES.
Just use your judgement about how much diarrea you reeeally need to have!
Oh yeah, drink LOTs of water today to make sure your hydrated well. The ES dehydrates you. And tomorrow start drinking water after your last ES drink...you may want to add a good probiotic to your diet afterwards to replenish the bacteria. (but I guess with ulcerative colitis you know what you can and can't do...that way?)
Good luck Maddie!
Hope you get out lots of stones and have amazing relief!