I'm pretty sure I got it correctly calibrated
Nope. In the case shown, the latitudes at least are plainly wrong. Double-check it.
Also, do not calibrate a homemade chart as anything except wgs84.
Is the original chart's datum compatible with WGS 84?
Old charts and maps can be drawn with some pretty eclectic local datums and will need shifting
to suit WGS84.
And, there's more possibilities beyond just getting the lat long correct- simplest-you need check that the original was DRAWN as a Mercator
, If not, the image must be reshaped
to suit the proportions of mercator's latitude vs longitude. This can be done pretty well in many image editors using the proportion of the central latitude's cosin. There's even a way to calibrate an image by interpolation from a single
"known" pixel but that's a bit involved...."Transverse mercator"..you won't likely need there.