If you're manually adding up Cxy, you need to pay attention to the definition as Cxy = -dQx/dVy, see the minus sign.

Spectre (and other simulators) support a CAPTAB option that can print the total capacitance at a node, but this will give misleading numbers if there is a small drain resistor (you have to find the capacitance on the internal drain of M2).

Another method is to put a small-signal source on the drain:
i2 (0 m2_D) isource dc=0 mag=1

then measure the small-signal (ac) voltage, and compute:
Ceff = imag(1/vac(m2_D)) / (2 * pi * freq)

where "freq" is the frequency where you measured the ac voltage.

This is because I = Y V, where the admittance Y is the parallel combination of the effective conductance and capacitance:
Y = Geff + j w Ceff
I = 1 (mag=1 on source i2)
Thus, Y = 1/V, and imag(1/V) = imag(Y) = w Ceff, so
Ceff = 1/ (2 * pi * freq) * imag(1/V)