Get Bot Margin to AddPage - FPDF

My PDF file is a table completed with values from a server.
And if the next line created is too large to display on the end of the page, I want to add a new page, then insert my new line.

I guess I must do some maths with $height_of_cell (which I calculate), getY() and margin-bottom but I don't know, and I didn't find how to get this last value.
Does anyone know ? Thank you !