Things I include that are easy to identify:
[my honest best estimate thinking off all I can think of] * X
+1 on the factor for any of these:
- always +1 right away (so it's x2)
- tech is new for you
- other teams involved
- you don't have a clear idea of what tasks are involved (it's not yet another X)
- you see additional risks
If the design is not pixel perfect or missing responsive: the customer is involved: x3
If you refactor stuff in to new tech and it changes apis to other teams: new tech, other people, risks, no clear tasks: x5