I was afraid someone would call me out on that, I just hoped it would not be within 30ish minutes. Yeah, I am terrible at the math involved, so I try to wing it. I did throw a Canada map in as my reference layer, though, for that very reason. This is the grid they used:

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Looks similar to what you use, at least to my very untrained eye.

As an aside, is there an easy way to convert that sort of thing back to Equirectangular?