Negatives and numbers bigger than 1 exist and are used on purpose in shaders. Correcto.

Tip: When we don't want a calculation to go outside the unit-range we "Clamp" it.

Math:Clamp is what you use.  In my notation,

Clamp(x) is always at least 0 and at most 1, by definition.

Mathematically the following is true:

when 0 <= x <= 1 , Clamp(x) = x
when x < 0, Clamp(x) = 0
when x > 1, Clamp(x) = 1

This is the definition of Clamp.

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