I don't think the adding of water matters much so long as you get the right amount of medication into your kitten. God bless you for being so caring as to looking after all those cats.
I will pray for you and hope it goes well. I am bad with maths, my mind boggles so I'm not much help.
I understand about the costs of vets, it can be simply too expensive and wish there was some way people could get help regardless of cost.
Please let us know how you get on.
Maybe in the future it would be easier with the 100 mg tablets as then you could probably cut them and then just administer as a pill???