Please pay attention to weighing cats when treating FIP. Above all, do not use cheap personal scales intended for weighing adults. These scales have a high degree of inaccuracy. I know a lot of people weigh a cat by weighing with and without the cat and the difference is the weight of the cat. Alternatively, you weigh the cat directly, but personal scales are not designed for such a low weight. So this weight may be far from the truth. The fact that a personal scale has a stated accuracy of 0.1 kg does not necessarily mean that it is also the highest weighing error. In fact, deviations of 0.5 kg or more are quite common with personal scales. All you have to do is stand on the scale differently, put it in a different place, or weigh yourself repeatedly, etc., etc.
It should be noted that a deviation of 0.5 kg in a person weighing 50 kg means 1%, but in a cat that weighs e.g. 2.5 kg already means a deviation of 20%. But it is the knowledge of the cat's weight that is crucial in determining the correct dose of the drug.
Therefore, for weighing cats, I recommend either kitchen scales or even better baby scales, which also have a fairly large area, and for a restless cat you will use the function of blocking the weight data so that you can read it at rest.
Especially young or very emaciated cats, which are expected to increase significantly during treatment, weigh regularly at least 1-2 times a week.
If you don't want to buy a weight for treatment, just go to the vet once a week and take the cat there. Many veterinarians even have a weight freely available in the waiting room.
And a little more advice. In any treatment, a slight overdose is better than an underdose. Therefore, if in doubt about the accuracy of the weighing, it is better to round up or use the highest weight from repeated weighing.
Example of a suitable weight: Baby Ono
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