It is easier with empty colon. Even easier with empty small intestine on top. Dulcolax will empty the first.
Miralax, is straight out toxic, even if the industry uses it everywhere as PEG. It is the main ingredient of windshield cleaner. Personally I prefer either (organic) coffee or sea-salt colonic - possible a home, too. Or tea like chineese "dieter's tea" or the Senokot tablets. Magnesium in any form, depending on the dose, is very effective too.