Most likely you'd see a deer at 50 yrds or a bit further. I was advised by my teacher to sight in for 50 yrds and if I need to shoot a bit longer distance just aim a bit higher.
You can also find ballistic tables for your ammunition on either a box, or on the manufacturer site (Winchester does it). That way you would know for sure how much more you need. My ammunition is +2" at 25 yrds, 0 at 50 and -5 at 75. I wouldn't use it for anything longer than that, but it's good to know that unlike rifles, shotgun slugs aren't that well designed for long-range shots. If you zero it in for 75 or 100 and you got a deer right in 25' from of ya, you risk shooting too high. 0 for 50 yrds gives it right in the sweet middle.