Please Google Bio-spot. It ISN'T safe for cats (and even some dogs!) It contains the ingredient permethrin, which is toxic to cats. Stick with something that is sold by your vet, that you've used before and know is safe for your kitties. A few dollars saved is absolutely NOT worth the life of your pet!
I use Advantage, because it is the only one that isn't contraindicated for cats with renal failure. I just buy a larger size and divide it between the three cats for cost effective-ness. All you need to do is add up the total weight of all of your kitties and buy the product for dogs that equals that amount and divide it between the whole crew. Cleo is 11 lbs, she needs 0.8 ml, Maggie & Lola are both 8.5 pounds, so they need 0.4 ml (total 1.6 ml.)
I buy the Red for dogs 11-20 pounds, treat my girls and take the leftover portion next door and treat their little Yorkie. I just draw it up from the tube into a 1 cc syringe, to measure it accurately.
Here's the amounts in each size tube (it's all the same product, just differing amounts):
Orange: For Cats Under 9 lbs - contains 0.4 ml
Purple: For cats over 9 lbs - contains 0.8 ml
Teal: For Dogs 11-20 lbs - contains 1.0 ml
Red: For Dogs 21-55 lbs - contains 2.5 mls
Blue: For Dogs over 55 lbs - contains 4.0 mls
Here's the link where I got the info http://www.entirelypets.com/eccon.html#Pookie