And to answer your question, no… 0.18 is not the value you want. And 50% is not a good starting point.
What we call the 50% perceptually gray patch is really just an approximation. And it practice it’s not perfectly 18% reflective either.
The OpenEXR spec says that an object correctly exposed that reflects 90% diffuse light should have a value of 0.9. So the best way to set the exposure for an HDR image (or any image for that matter) is to set the values of the color checker chart to their real reflectance values.
Of course, this works if the color checker is lit in a way that represents the entire image.