Hi, welcome to the forum.
The envelope look ok in the picture, where is the problem area.
If you are not getting straight lines with the bezier tool there are a couple of things you could do. When you start the line hold down the control key while yor drag your line out.
turn on the grid Andrew along the grid line, ensure that snap to Gide is on.
Sorry I am not where I can get to my computer to give inst on where to fin the grid on menus but I think under file, but somewhere in the top menu.
Someone will come along soon I am sure to clarify what I have just said
LEARN, GROW, CREATE, SHARE
Mac 10.9.5 Yosemite, eCAL 1.506, ECAL2.023, SCAL4Pro, Export Plug-in, Inkscape 48, iPad/scal3