It sounds, as described, like a futile exercise (not a "hard" question, just more of a "what difference does it make" question), but if you wish to go forward with same, I would sight to zero (X-ring) at 15 feet and call it good (same zero setting) at 75 feet, assuming you are shooting 22 longrifle ammo. I don't believe you will experience much trajectory variance over the intended ranges.
I'm not even sure your bullet will have completely stabilized at 15 ft. That's barely 5x the barrel length. From all I remember about bullet trajectory it's still stabilizing out till nearly 10 yds.
I'm with ****. I'd be much more concerned about the horizontal aspect of your accuracy at that close a distance.
Another concern here. Are you thinking of sighting this in at an indoor location?
There is high risk of lead and gunpowder contamination, and possible poisoning at any indoor location without specialized air re-circulating equipment.
If you're considering sighting your rifle in your basement, I highly recommend that you don't do it. Other dangers are ricochet and a possibility of fire.
Always put safety in front of any shooting session.