Right - American English favors "toward," and UK standard tends to add an 's.'
This also applies to "forward," "backward," etc. These are explained in more detail by Fowler, Merriam-Websters Dictionary of English Usage, et al.
If you are trying to communicate clearly, in general it's best to avoid Latin phrases with some exceptions.
First, common Latin abbreviations (etc., e.g., i.e., et al.) are perfectly fine. Similarly, do not hesitate to use Latin phrases that are so common that they are rarely thought of as being a Latin phrase, e.g., status quo, rigor mortis, and per capita.
Also, do not hesitate to use Latin phrases in contexts where they have precise technical meanings. For example, lawyers and academics will use phrases like de facto, de jure, quid pro quo, a posteriori, and a priori with no potential for misunderstanding among professionals.
In summary, if you did not know the meaning of a specific phrase until recently, it may be likely that many other people do not recognize that phrase so its best to avoid it.
Best Answer
I would substitute an adjective that means the same thing: totally, completely, absolutely, etc.
As it stands, though, your question makes no sense: "cent percent" is meaningless. Are you asking whether you should spell out "one hundred percent" instead of using numerals? If so, it probably doesn't make a lot of difference. Still, in the context I would use one of the adjectives I suggested here.