Ken lad, why not give it a go? Mine works a treat. Just get the feeblest red glow inside the finder... just enough to make out the X hairs against the objective lens. If it's too bright, simply increase the value of the resistor to make the LED dimmer.
When I modify a Telrad, I use an LED & 100k resistor to show when power is applied. The LED only gives a very soft glow. That is all you require.
HTH.
L.
ps. What sort of finder are you using?