@Frenzie
To report success!! 
Sorry for knocking insistently at your door.
I tried your patch, inserting manually these two lines with 50 value and
it fits exactly. The display is perfect (at least on some pages of the trial ePub) with the same amount of lines per page it is intended to have.
As far as I am concerned, I shall keep it this way until further notice and shall test further this new muPDF ePub renderer. I'll need some days for it and shall report back here asap.