Titanic had, IIRC, more of a red lead paint scheme. Which would probably want mixing Flat Red with Medium Gray--how much will be by eye.
Your references will be a better guide than some absolute formula.
And, there's "scale effect" to consider, as well. So, your color needs to be a bit paler at 1/350 than at 1/144 or larger.
A person can be easily forgiven for just shooting a coat of Hull Red on--after all, no one is likely to pop up with digital photos of the launching to prove you wrong.
It's rather a bit like arguing over which black to paint the hull (me, I'd use NATO Black for scale effect, and to pe able to pick out details using "straight" black).
My 2¢; you local currency may differ.