If you want to know where 18% reflectance will fall on a BMDFilm Log curve you can have a look at the VFX IO LUTs in Resolve. Open the Linear to BMDFilm 1D LUT with a text editor that numerically lists the rows (something like BBEdit). It's a 12bit LUT so has 4096 points, and the BMDFilm Log range is listed as -0.0071215555 to 5.7661304310. That range is mapped to the range 0.0 to 1.0 by the LUT. With 5.7661304310 mapped to 1.0 (point 4096), 1 is mapped to point 710, and by referring to the Lookup Table the corresponding new value is 0.6769, which if you apply the LUT to a grey scale ramp you'll notice is where 100% IRE is mapped to.

If you meter to 18% reflectance, that becomes 20 IRE when viewing a waveform (90% reflectance is maximum reflectance so is mapped to 100 IRE, therefore 18% is mapped to 20 IRE). If you want to know how other reflectance values are mapped, then divide 710 by the value and refer to the Lookup table. 20%(18% reflectance) is at point 142, and the corresponding value is 0.393522.

My math might be a bit sketchy, but hopefully you get the gist of it.