Organic Share

Hi, everyone!
Can anyone share their methods how to calculate the organic keyword position share compared to competitors? How much the client has market share in organic results when taking in account all niche relevant keywords.

Here is something I came up with to calculate maximum organic potential:

Keyword planner research + webmasters tools queries = maximum search potential including adwords share
Maximum search potential * 0.3 = maximum organic potential //I took 0.3 because that's somewhere close the average CTR for the 1st position in organic results

My calculations are for sessions however I am interested in percentage when compared to competitors.
I hope you get my idea, I am open to listen your suggestions.

I swear I do not work for Semrush (second time I recommend them here on BHW today) BUT, you can do this v quickly and easily with that.

Unfortunately with semrush we would reach the pages to crawl limit really fast because I represent SEO agency and we have few large ecommerce clients in addition, there are clients which could single-handedly reach maximum number of crawled pages. That is why I am looking for simple solutions which could help me to calculate this.
Btw I am using Google speadsheets consistently if that helps.

SEMRush offers an API option - perhaps they might be able to offer your agency a bulk solution.. reach out to them.

the organic keyword position share compared to competitors? How much the client has market share in organic results when taking in account all niche relevant keywords.