How to determine spline size without old spline

Hi Guys,

I've got an old sliding patio screen door that needs the screen replaced. The spine in the door now is not the original, and is too small to properly hold the old screen. I replaced the screen and spline years ago and matched up the original spline to what was at the hardware store. I think I bought a replacement spline that was too small, as the screen has come out a few times over the years.

I've looked all over the internet to find how to determine the correct spine size........but every article says to just bring the old spline with you to the hardware store. But what if you don't have the old spine (or it's the wrong size)? The grove the screen and spline will sit in looks to measure 0.20 inches, but a closer look shows that one side of the channel actually expands once past the initial lip and is 0.33 inches. From the side the channel would look like an "L" more than a trench with parallel walls (I've attached a pic of the corner). There is no label on the door, but the sliding lock has a P over D logo on it.