How many pixels the "normal" letter-spacing is?

We are working on an input-field in our application where the users can adjust the letter spacing of text elements in pixels, but we are not sure how many pixels the normal letter spacing is.

We would need to make it possible for them to reset the actual value to normal by inputting pixels.

This is what the input field looks like in the application.

Letter spacing:

[___] ↕ px

Should we default back to normal if they reduce the letter spacing to 0 px? Or 1 px?