Final word on measured dosing of Advantage/Frontline?

I asked in a previous post whether indeed we could save money by buying the
largest size of Advantage or Frontline Plus (for the largest dogs) and then
accurately measuring out a smaller dose for cats. The consensus was that
this is indeed possible, and for example, a single "large-dog" dose of
Frontline Plus could provide enough medicatioin for 8 cat-sized doses.
There's clearly no problem if there are 8 cats available to take their dose
at the same time. But what if you only have one cat to treat and you want
to save the other 7/8 of the medication to be used at a later time? Can the
unused liquid be stored in an airtight container without losing its strenth?
I called Frontline's 800 number to confirm that you can indeed treat your
cat with a small measured portion of a "large dog" dose and save money in
the process. She was discouraging of the practice, saying they really
didn't recommend it because you had to have a means of accurately measuring
out the dose and the pre-packaged cat-dose was the only way to really be
sure. I had the feeling this was the official instruction on what they were
supposed to say because obviously the company would like to sell more of the
single-cat doses for more revenue. But when I pressed her about it, she
acknowledged it could work, although she did say there was a slightly higher
concentration of one of the active ingredients in the cat's formulation than
in the dog's. When I pressed her further, she admitted that slight
difference would probably not have a major effect on the efficacy of the
product. But she did go on to say that storing the product might be a
problem. It has no expiration as long as it's kept in its air-tight
container, but when it's opened and exposed to air, it loses its strenth.
At this point, I don't know whether to believe her or not! So I invite the
comments of anyone who knows for sure. Can an opened container of Frontline
Plus be stored without losing its effectiveness?
I didn't call Advantage to ask them about their product. I figured I'd just
ask here. So does anyone know how long Advantage can be stored, once it's
opened? I figure the answers I get here at least won't be colored by the
profit motive!
Also if anyone has any strong feelings as to which is the better product,
Advantage or Frontline Plus, please share them.

Thanks for the information...

-Garret Swayne
garret (at) garretswayne (dot) com