Excel WSSI

I have an excel wssi that looks at sales, stock and intake by week.

I also have an Open To Buy variable that calculates the difference between my target stock and my current stock by week.

I then have a variabel called "Trigger Dates" that allows me to enter an "x" on weeks where i want intake.

What i need is for where an "x" is inserted, the OTB values are summed until the next week with an "x".

Therefore if i have OTB vales as follows;

wk 1 = 100
wk 2 = 100
wk 3 = 100
wk 4 = 100
wk 5 = 100

and i place a trigger date "x" under weeks 1 and 3 then my OTB values should be 200 for wk 1 and 300 for wk 3.

I have tried to use a sumif calculation but i do not know how to make it only sum between "x" and "x".

The "x" triger dates need to be able to be changed and so the OTB calculation to change accordingly.

Hope this makes sense?!

Can anyone help?