Roger that, TorpX.
An easy way to test, at least, it's what I did with SH3 was use the "in game" ruler tool to measure the distance between Meridians and also between Parallels. It doesn't stack up to a real Mercator projection chart.
So most of the time I make my own (Paper) 2° x 2° charts to annotate my progress using Mary Blewitt's instructive book, CELESTIAL NAVIGATION for YACHTSMEN.(about $13.00 US) (Still Avail via Amazon via Subsim). I highly recommend her book. During WWII she taught many Navy and Airmen how to nav.