At HTML Select elements there is around 4px indent.
How can I calculate/ get the exact value using JavaScript
I tried this but doesn't get me the correct value
const el = document.querySelector('select');
const computedStyle = getComputedStyle(el);
computedStyle.textIndent // '0px'
computedStyle.padding // '0px'

I would try to determine based on CSS box model. In your case I would expect it to have a
pading-left. You want the space from the select border to the text content. This is affected by the select padding, the option margin, the option border and the option padding.