My baby's pediatrician said since she's on a soy based formula she would get constipated, he said I…

My baby's pediatrician said since she's on a soy based formula she would get constipated, he said I could give her a 1/4 capsule of Miralax, but I never found the capsules, only the powder...does ANYONE have any idea the equivalents of a 1/4 of a capsule to just the powder form? I've looked it up but couldn't find any answers.