I agree with Chris and Wendy - I think it would be ok to go to the 16-17u range. It was a fine choice to give a token 10u tonight, but it does look like the 16-17u is a pretty good dose.
To say a little more about what Wendy's saying, your hours count is always counting from the last shot. It's not counting from when you ordinarily would shoot, but always how many hours it has been since the last shot. If you go past +12, you just keep counting +13, +14, +24, etc.
Just brainstorming about the megacolon/constipation issues . . . There is a good link about IBD (Irritable Bowel Disease) kitties and probiotics, and there is a second good link about cats that vomit a lot. My cats don't have megacolon, so I'm uncertain if this would be helpful for Menda or not, but I'll give you the links for you to look at and decide if you want to try it with Menda.
Digestive Problems:
Constipation, Maggie11 & Ann; Feline Constipation
Hairballs/Chronic Vomiting
Probiotics in IBD (or other) cats
My short haired cat, McGee, was vomiting daily and also very constipated. Those links suggest that this can be from a cat having very slow motility of their gut, causing them to both vomit and be constipated. We followed the directions in the probiotics link, giving him the Renew Ultimate Colon Care 80,000 cfu dose. We started small and took a couple of weeks, gradually increasing the dose until it was 1 capsule per day. Then we gave him that 1 capsule per day for 2 months before moving to a maintenance dose of one capsule every other day.
We also give him a tiny sprinkle of ground flax seed. You want people-quality seed. I started with just a few grains, very tiny amount, then worked up to slightly more. It's still smaller than the amount that a coin like a dime would cover. I just kept increasing a little bit until his stool was normal consistency instead of rock hard.
You might want to ask your vet if the flax seed would be ok to try for Menda. It's effectively adding fiber and bulk. Some people give canned plain pumpkin to provide that bulk/fiber.
Having a high dose cat keeps you on your toes, doesn't it?! How is Menda doing otherwise?