When I put the gird on it doesn't match the chart, I get waves in the grid lines, after activating the changes
means you don't have the calibration points' lat/lon correct.
you are calibrating as the wrong projection- I mean, you must know if the image is MERCATOR, Transverse Mercator, Universal Transverse Mercator and so on.
the image you are calibrating is NOT an accurate projection at all and is not intended for navigation
. .. for instance, a gas station road map is about showing roads plainly...likewise a Rutgers Sea Surface Temperature map is about showing temperatures, not for gps navigation
That said, Your "best bet" for most maps is MERCATOR, unless you are closer to the poles.
(The image can sometimes be correctly resized to approximate a correct projection per its latitude)
As to "Scale", without doing some math here, try a simple way that might take a few tries... view your map image at a scale where it is still plain and not a bunch of useless pixels on your screen
Compare to a similar chart covering the same view
and note the scale Opencpn
displays in the lower right. This is a good starting
point ..just a wild guess but it'll likely be something like 2500000 or at least 1000000 unless they are so kind as to offer very large and plain pdfs for free