So Colace doesn't seem to be working so well and I'd like to switch to Miralax, thank you everyone here! How much should I start with? Any helpful suggestions? My LIS is on Thursday, so I really need to get this resolved.

It might take awhile for you to know because everyone is different. You might try with one capful (sachet) and adjust as necessary. I take 4 teaspoons daily (divided).

I use the capful.

I started with one capful (17 grams) as well. That's probably the safest bet. Once you see how it works for you, you can try adjusting the dosage accordingly. Bear in mind that Miralax can take a day or two to take effect, so if you don't see much effect right away, don't get discouraged. Be sure to drink lots of water while taking it because that really helps it work better.
Nowadays, I'm a "Miralax vet" and adjust the dose depending on what I ate during the day: if it was a McDonald's kind of day, I use a full cap; if it was a veggie soup kind of day, I go as low as half a cap :)