I can't say a specific dosage of psyllium per body weight, but I was told some years ago by my vet to open one psyllium husk capsule and sprinkle the entire contents over my cat's (wet) food twice a day. My cat was 20 lbs at his heaviest. Now he is about 15 and a half pounds, and another vet at the same practice said I should continue that same amount, as I've been doing.
The psyllium husk capsules I use say "100% psyllium seed husk." There are other ingredients such as gelatin, etc. The label says that one serving (6 capsules) provides 2.4 grams of soluble fiber.
There are other very similar-looking bottles that have psyllium husk but that also contain other medications for constipation, intended for humans. I would be very careful about reading the label of any psyllium husk product you buy, and would stick with the plain 100% psyllium husk unless a vet tells you otherwise. The bottles look very similar, and even after years of buying it, I have managed to get the wrong product and had to return it.
I buy the plain psyllium husk capsules at Walmart and Walgreens, and I buy the store brands as they are cheaper than the big-name brand (Metamucil).
I use a pair of scissors to snip off the end of the capsule over the sink or wastebasket, as a little bit of the powder puffs out when I do it. I mash the powder into the canned food and mix it up a bit and my cat doesn't seem to notice it at all, but then, he's never been a super-picky eater.
If your cat doesn't eat all the food within a short time frame, you might want to throw it out, as I don't know if the fiber expands over time when it comes in contact with moisture, and could perhaps cause a choking hazard.
If you're giving it by syringe and mixing it with water, I wonder if you'll be able to see if it starts to "gel up," which I would think could pose a hazard. I'm thinking maybe you will be able to tell visually that it's thickening, and that it needs more water added.
Hope this is helpful,
Margaret