Is It Ok To Write "100%" In A Formal Text?

I'm writing a text that is neither mathematical, nor statistical, but rather I want to emphasize a point. For example, I'd say "It's 100% involuntary..."

Is it acceptable to employ "100%" in normal nonfiction writing, as a synonym for absolutely or completely?

The statistical-sounding expression is no problem, but if you want to be formal in register, as your headline indicates, you should probably spell it out as “one hundred percent”—and also spell out “it is” (which latter you have here contracted).

I'd use a word instead of "100%" since the value is not meaningful...try "it's involuntary" or if emphasis is needed, "it's absolutely involuntary" or "it's entirely involuntary".

The type of writing you are doing also plays into your decision. For example, in legally binding documents, like contracts or exhibits to contracts, the spelled out number is the legally binding number. So if a text said that, "you are 99% (one-hundred percent) responsible", the 100% number would be legally binding, not 99%.