Well I would follow the instructions as best you can. I know with some colon cleansing programs, you can bend some of the rules slightly and it won't negatively affect the outcome. But I have never used Cleanse Smart, so I can't tell you how strict their regulations are.
I would suggest that you call Renew Life and speak with their customer service representatives. See what they have to say. They will know before anyone on this forum would.
or, you can just do like the last guy said and eat your popcorn a few hours earlier.