Here's how you can figure it out.
1. Shoulder your rifle with whatever stock you're going to use, with your normal cheek weld.
2. Either measure yourself, or have someone else measure, the distance from your eyeball to the rear of the flat-top rail. Call this "EBR". (eye behind rail)
(an easy way to do this without a helper is to shoulder the rifle, keeping the rifle held up in position with your strong hand, use your weak hand to touch your cheekbone just below your eye; drop it straight down to the stock and mark this position)
3. Take the opimum eye relief for the scope you're using. This is published by the manufacturer. Call this OER (optimum eye relief).
4. Call distance from the rear of the rail to the ocular (rear) lens of the scope OFR (ocular front of rail)
You want OFR + EBR == OER.
In other words, you want the distance from your eye to the ocular lens to be the optimum eye relief published for the scope. This will be real close, and if not exactly on, it'll be a good starting point.
For example, a TA11 has 2.4" eye relief (OER). I shoot nose-to-charging-handle which puts my eye about 1.5" behind the rail (EBR). So I have to put the ocular lens of the TA11 about 0.9" forward of the rear of the rail (OFR).