symbols - indefinite article before I(0)

I'm writing my thesis and need to write: "[...] and a/an I₀ of [...]" and am puzzled about which indefinite article to use.

I(q) is a symbol for intensity as a function of the momentum transfer q and I assume the indefinite article before intensity would be an. I₀ is the intensity where q=0 and is also known as the "forward scattering", and I would assume the forward scattering would have the indefinite article a.

So dependent on whether you see I₀ as the forward scattering or the intensity at the point where q=0, I would use a different indefinite article. I read it in my head as forward scattering so the an "sounds wrong", but when I look at the page I think that the a "looks wrong". Are there any defined rules?

Thanks in advance and please bear with me as English is not my first language.