There is no regulation that requires factoring.
True for you perhaps, but lack of regulation does not mean, do what you like.
Would you fly with 0% variable and 0 minutes fixed fuel reserves, if there were no regulation? What would guide you in your decision on fuel reserve, if there were no regulation?
We still have a duty of care. We all know that we are not all test pilots flying in controlled test conditions. We know that, and the court of inquiry will know that too.
How do you assess it, if not by reference to factors
implied by regulations as being a suitable standard?
Do you use another method? One that can stand scrutiny?
I ask out of curiosity, not rhetorically - factors are treated as in-flight requirements as well as pre-departure requirement over here.