@Manuel_Angel - I went basically the same way 
. Just in case someone is interested in this, here’s an estimate of the transfer function from illumination levels (EVs, x-axis) to 8bit jpg output values of the HQ camera (y-axis; you need these gain functions for real HDR work):
The curves are centered by my software always at the data point 0.0 EV and 128 output (thus they match at that point).
Well, not to bad a response curve for the HQ camera, and the difference in linearity between 128 and 200 is certainly negligable with the HQ camera. That is however not always the case. For example, the same curve for v1 Raspi camera looks like this
so this camera behaves differently at intermediate gray values (~128) compared to operating at brighter gray values (~200).