I also recommend to re-orientate the image so that the chirp-grating is either horizontal or vertical. (For the sample image I get a rotation angle of about 16.1 degree to obtain horizontal grid-lines.)

Now project the image either horizontally or vertically. What you get is a plot like this:

From the local maxima you get the coordinates of the gridlines, i.e. you can position the line selection appropriately and do the desired measurements.
Please tell us if this is of any help for you or if you need further help.