EDIT: After all my blathering, I caught your additional details. LOL Even so, I think it will still be ok if you have to serve some dry. I don't think it has to be special dry either. What I would do in your case is to feed a wet meal twice per day before you leave and when you get back, but do leave some dry out for nibbling between times.
For the times when you're away, I think it would be ok to leave him on the dry. If possible, perhaps he could get one last wet meal before you leave, but even if it's only two days' worth of dry meals, I don't think it would be all that bad. Or better yet, would it be possible to have someone stop in to feed him?
It's just barely possible that you could leave a canned food meal out for him....put the bowl on top of an ice pack. That would preserve it for a little bit longer. But I'm not sure that it would be worthwhile.
Although I agree with that statement, it's not the end of the world if you do feed some dry food. Obviously, the better quality the better.
In your case, perhaps it's necessary to let them keep eating some dry.
But to literally answer your question: The same rules apply and they're actually pretty simple. Starting with the simplest, you could feed the suggested amount from the can or bag for your cat's ideal weight. Split that into as many meals per day as you plan to serve and of course if you're feeding a variety you have to take that into account as well....if you feed half dry and half canned then you'd obviously feed half the suggested amount.
But also remember that those guidelines tend to be a bit generous so you might feed slightly less and/or adjust for your cat's appetite. Also remember to feed for the cat's IDEAL weight which may or may not be its current weight.
If you want to get more technical about it: Find out how many calories are in the food(s) you're using. You'll probably have to go the website to find that out. Then determine your cat's ideal weight. Note its activity level. Decide if weight loss is desired.
Pick a number between 20-30 that represents the activity level. 20 is weight loss or couch potato, 30 is maniac. Multiply that number by the cat's ideal weight to get how many calories it can eat per day.
I think you can figure out what to do from there.
For me it's pretty simple. For Sophie who doesn't have weight problems, I just feed her varying amounts based on her appetite. On a hungry day, she'll put away 1/2 of a 5.5oz can in one meal. On slower days it might be 1/4.
For Poppy who does have weight issues, it's more difficult because raw meat doesn't come with calorie content. I've had to experiment a bit there and have settled on a total of 4oz per day for her.
Our schedule is pretty normal - feed at 6am and again at 3-4pm when hubby gets home. We're home on weekends. If we're away, my MIL comes down and takes care of them.