I'm about to begin shooting a short film where there will be a scene in which there is a spotlight on the subject. It will be outside before dark (at the "magic hour"). Also there will be some at complete night with just the spotlight. I'm also gonna be using a yellow or red filter. I was wondering what would be the most acurate way to get a light reading. I was thinking of using a 18% grey card.I'm gonna do a test roll first on Tri-X, so i wanted to get some suggestions.
I'm also not sure if i wanna use Plus-X or Tri-X yet. I think i'll end up using Tri-x because i want some white objects to have a little overlit feel. The only problem is there'll be a white wall behind the subject which i don't want to be too bright, that's why i need an accurate light reading. Any help or advice is appreciated.

-Ivan