optics - Optical power through a circular aperture?

I have the following situation to evaluate:

A laser emitting a Gaussian beam having $d_{63}=5 mm$, a small divergence $\theta$ and power $P_{0}$.

$d_{63}$ is the diameter in which my beam has 63% of its power.

I need to compute the power through a circular aperture of diameter $\phi=7 mm$ placed at a distance $z=100mm$ from the laser.

I would apply the formula for finding the % of power which pass the aperture:

$$T=1-e^{-({\frac{\phi}{d_{100mm}}})^2}$$

Where $d_{100mm}$ is the diameter of the beam computed as $d_{100mm}=d_{63}+r\theta$

And after

$$P_{out}=T*P_{0}$$

Heres my biggest doubt: in the formula for finding $d_{100mm}$ is supposed I know the waist of my beam at the origin ($z=0$, the laser aperture in this case) but I don't actually know it. I only know the diameter where the power is 63%. So I think it's wrong. How do you think about that? There is a way to correctly apply this formula in my case? Also, which are the differences if I know the 86% diameter instead of 63%? Many thanks.