Comment by bryanlarsen

That advice makes more sense according to my understanding of the physics -- the entire partial might be slightly off and every position in the partial should be adjusted similarly.

The just/equal temperament thing lead me to suspect that it was the 5th partial (a major 3rd partial, the D) that would be the one most likely to be off, but a trombone is neither a perfect cylinder nor a perfect cone so simplified models might be off. The perfect 5th (aka both the F partials) is pretty exact on an ideal model, but a real trombone isn't ideal.