calculate running sum

Hi,

Env: HANA SPS6

I want to create a flag column in an existing HANA model that will show if a record is in the bottom 20% of a measure value, to do this we need to perform a running sum of this measure value and then perform a if statement to say if its 80% of the total population, could you help me with an idea to do this?

Many thanks

Avi