LTSpice AC Simulation giving unexpected output
Rser (10 kohm) is the dominant resistance so use that in your calculation. If you want a more precise calculation Use 10 kohm in parallel with R2 (about 9 kohm) to calculate the cut off frequency.
$$\boxed{f_C = \dfrac{ 1}{ 2\pi CR}}$$
Added Section
So, if I do a simulation and use the DC gain as a reference I get this: -

The left cursor is positioned at an arbitrarily low frequency (1.14 kHz) and this shows that the DC gain is -0.827 dB as predicted in the comments. If I then move the right cursor so that the &quot;delta&quot; is 3.01 dB, the frequency is shown to be at 1.094 MHz. This tallies with the equation I wrote last night when you use the parallel resistance of the 10 kohm and the 100 kohm. The parallel resistance is 9.0909 kohm.