Stringgrid automatic rowheight without header height

i want to make  automatic rowheight

i make this Stringgrid.RowHeight:=(trunc(Stringgrid.Height)) div Stringgrid.Rowcount;

but including header height how to eliminate the height ?

Stringgrid.RowHeight:=(trunc(Stringgrid.Height)-[Header height]) div Stringgrid.Rowcount;

thanks

other question Stringgrid.option := [] how to make header to false (sorry for my english i'm french);