Is a "drop-finesse" better than a simple "drop" play with a nine card suit in bridge?
Your calculation seems off. You seem to have missed an approximately 5% chance of a 4-0 break which the play for drop will catch, but have that implicitly included in the calculations for the drop-finesse.

If you include that, the play for drop indeed comes out to be more, but by very little.

A possibly simple way to figure this out is to compare the holdings where one wins but the other loses.

Say you have AJT9x opposite K87x (in hand) and you decide to play the K first (in both lines).

Taking the finesse next wins over the drop when RHO has Qxx (assuming both follow to the first trick, otherwise it is the same for both lines).

Playing for the drop wins over the finesse when RHO has Qx.

Since RHO will have Qxx 3/4ths of 1-3 split = 3/4ths of ~25% = ~19%, while RHO will have Qx 1/2 of ~40%  = ~20%, playing for the drop is better.