Hi Marcy,
Cody gets miralax in every meal because he has bad megacolon. Miralax is one of the first wonderful suggestions I got on FDMB. I think it honestly saved his life.
I use a measuring spoon and add a level 1/8t to each 1/2 can of 5.5oz Friskies (or 1/4t per 5.5oz can). Since some cheaper foods have more water, if I feed those, I cut back on the dose.
It is important to increase the cats water in addition to adding miralax, because it works by drawing water into the colon, so i expect it could make them dehydrated if you don't increase water. Many add water to the food. I give fluids and add water to the food.
Miralax works slowly, so if he is blocked up, that may need to pass before you see softer poo.
If it gets too much, the poo just oozes out uncontrollably. Its a pretty delicate balance so it would be good to measure, and stick with a dose for several days until you get it right.
The added water helps Cody a lot all by itself. I'm thinking high numbers tend to dehydrate a cat also. the kidneys lose water as they try to flush sugar.