I've stopped tracking this as much as I once did, but the last at-large spot in the NT for D3 generally falls in the mid/upper 40s (I've seen it be as high as 42 in the projection report, and as low as 52), At D2, with fewer conferences, that spot usually goes to a team in the mid/upper 50s. I've even seen No. 60 get into the NT previously. Sorry, haven't paid a lick of attention to D1 on the matter. (all numbers listed above indicate a team's Projection Report position, NOT the RPI)
Keep in mind the Projection Report keeps updating through the final round of conference tournament games, so if you're on the borderline, it is possible to be leapfrogged at the last second based on the results of the championship games in the conference tournaments.