Hate to break it to you, but you just define rounding down.
Back to the OP. The below math is in excel so I am not 100% on accuracy, but close. Assuming an Orca with T2 rings, T2 Drones, and all lvl 5 skills except Mining Drone Spec.
Each drone will mine the following UNITs of ore by skill level;
Veld; 1 - 2650, 2 - 2702, 3 - 2754, 4 - 2806, 5 - 2858
Gniess; 1 - 53, 2 - 54, 3 - 55, 4 - 56, 5 - 57
16m3 ore; 1 - 16, 2 - 16, 3 - 17, 4 - 17, 5 - 17
16m3 or includes spod, crock, bist, and ark.
EDIT: I was applying rigs incorrectly, but with the new numbers, Mining Drone Spec stops being useful at lvl 5 only with 16m3 ore. Every other volume ore Grant’s you at least 1 more unit per cycle.