Calculate story points capacity and display result in a custom field

[KRC]

Background:

In project XYZ:

For every story & task, we have story points.

I want to calculate all story-points assigned to stories and tasks assigned to Mark in the current sprint(Capacity Demand) and calculate it against a numeric field called Planned Capacity and display the difference in another numeric field called Free capacity

Capacity Demand - Planned Capacity = Free capacity

Capacity Demand= All-Story points of an assignee in the current sprint in a given project

Planned capacity = a numeric custom field, in which values are entered manually

Free capacity = a numeric custom field, where difference should get displayed