Calculate velocity through weeks

Hello I've been trying to solve this puzzle without any luck.... :smileysad:

What I need is to calculate the velocity per week, in the  excel table below for instance, on Period1 the velocity is 1.395   =I21/(I20*4.3)

then on Period 2, since there is no sale, the velocity is calculated =(I21/J20)/4.3 last month sale  vs the sum of periods 1+1=2 periods instead of 1, period 3 is the same 6 PKG, per week divided by 3 periods 4.465... and so on.

My problem is that I can't figure out how to create the measure that go check if there is sale on the period, if not go back until it find a positive value, then sum the total amount of periods and divided by thePKG per week.

Your help will be greatly appreciated!

Thank you!