To give a quick, approximate, answer to the first question (see below for the significant caveats), the relevant figure of merit is the detectable chirp mass for a given configuration defined as $$ M_\textrm{chirp} = (1+z) \left(\frac{M_1M_2}{M_1 + M_2}\right)^{3/5} (M_1 + M_2)^{2/5},$$ where $M_1$ and $M_2$ are the masses of the merging black holes an $z$ is the redshift of the black hole. The sensitivity of a given experiment to a given chirp mass will depend a lot on the noise properties of an experiment. For an example for Advanced LIGO, see here: https://core.ac.uk/download/pdf/267293216.pdf
We can do a very rough approximation for the inspiraling phase of a merger fully classically, assuming no relativistic corrections and no black hole spin. The specific derivation can be found here: https://arxiv.org/pdf/gr-qc/9402014.pdf but in the end we get a differential equation relating the chirp mass to the frequency evolution of the merger, $$\frac{df}{dt} = \frac{96}{5} \pi^{8/3} \left(\frac{G M_\textrm{chirp}}{c^3}\right)^{5/3}f^{11/3}.$$ We can use an extension to newtonian gravity to understand the merger better as we start going into the relativistic regime (so called post-newtonian formalism). 675
Derivations related to using post-newtonian formalism applied to black hole mergers can be found here: https://arxiv.org/abs/<PHONE_NUMBER>
The general solution for the actual observed frequency is a very complicated formula it depend on a lot of factors of the merger.
1) The mass of the black holes 2) The redshift the merger occurs 3) The angular momenta (i.e. spin) of the incoming black holes 4) The orbital eccentricity of the system 5) The orientation of the merger 6) The relative angle between the arms of interferometer and the incoming gravitational wave
In addition to this, there are multiple regimes of the black hole merger; inspiraling, merger, ringdown. In the case of current black hole detection, we don't look for single individual spikes but try to find an entire waveform. There are multiple templates for various events that are generated from numerical simulations which can be used as matched filters to try to find merger signatures. 675