Help with ratings column

I have a composite custom column #read_count, its template is {#date_read_history:count(,)}

I want to have a custom rating column (#my_rating) that is recalculated whenever #read_count changes.

My understanding is that the underlying value of a rating is an integer between 1 and 10. The attached table shows how I'd like the #read_count column to be used to define a value for #my_rating. I assume I need to extend the #read_count's existing template to compute the value of #my_rating - but I have no idea how to code that.

Could someone help me with it please?

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