How far back shall I go when calculating the average movement?

How far you go back on news trades depends on the report you are looking at. Some reports are released every 4-6 weeks whereas others are release twice a month. 

Please revise over the 'online course' and the 'mentoring videos' if you are unsure about how far you need to go

https://www.traderess.com/products/online-course-stage-1/categories/2642323/posts/8818690