With 440 Hz at middle A (midi note 69) you get:
Hertz = 440.0 * pow(2.0, (midi note - 69)/12);
Reversed:
midi note = log(Hertz/440.0)/log(2) * 12 + 69;
If you want to play the correct frequency from that, you need to pitch bend from the midi note. Say you want 430.3 Hz when having +/- 200 cents pitch bend:
1 cent = 1/1200 octave
log(430.3/440)/log(2) * 1200 ~= -39 cents
with +8191/-8192 resolution of pitch bend that would be:
pitch bend value = 8192 * -39/200 = -1597
Hope that helps…