Derivative sum - Elasticsearch

Hi guys, i have been tasked to create a dashboard which shows disk storage capacity predictions. Our monitoring support are using elastic to store metrics.

I created a query that would map the disk usage over the last 30 days and then also a query that would give the derivative of 90days. However the derivative is not including the current usage.

So if current usage is 85% and in the next 90 days its going to rise by 10% i need to be able to have a query which we can alert on which is 95%.

The need for alerts is our solution to having 1000 servers with lots of drives but our capacity team only needs to know of the drives that are going to be full or near full in the next 90-120 days. so i figured i would script creating the dashboard with all servers and drives and then have at the top an alerts panel which would only show the drives that meet the above.

Unless there is a better way of achieving this. Id like to avoid having to pull monthly/daily metrics into an influxdb if i can avoid this.

Thanks for the help.

Elastic version: self hosted opendistro
Grafana: Grafana v7.3.7