2002 Ford Focus

Okay I'm not sure I understand exactly what you're fixing to do.
Are you talking about adding just anti-freeze to your radiator or a 50/50 mix?

Adding too much antifreeze (not the mix) is not necessarily a good idea because antifreeze doesn't soak up the heat from your engine as well as water. So your car could overheat if you do that.
If you're determine to strenghten your coolant don't go beyond a 70/30 ratio. Honestly 50/50 works just fine in most situation; it has a freezing protection down to -34F and a boiling protection of 255F.

If you're not sure about your coolant and want to check its strength, buy you a tester ($2-3).
Finally, adding antifreeze directly to your overflow tank is not a good idea because you won't be able to know what your ratio is; you should mix antifreeze with water in a separate container and then pour it in your car.
If you're worried your coolant has depleted, adding antifreeze is not the solution. You would need to service your cooling system, have your system flushed, and put fresh new coolant.