Ok, as a forum noob, I'm not sure if this is a tutorial per se, but I just created this for my new map and I thought I'd share, because it took me hours of research to figure out how to do it.

What I was trying to do was figure out where the latitude lines should be in a Mercator map of a world that doesn't exist (yet). I chose Mercator for the same apparent reason that Google did -- it's zoomable as-is, without having to reproject a continent (or piece of one) before you do a blow-up version of it.

So, if you pick a circumference for your world, and a base scale (scale at equator), and a cut-off latitude (70-85, usually, depending on how much distortion you want to put up with), this OpenOffice spreadsheet (the zip contains both .ods and .xls versions in case you have Excel) will calculate where to put latitude lines, how big (in pixels) your map should be total, and what the scale is at each latitude for your zoomed-in maps.

I hope this helps anyone who's ambitious (or crazy) enough to want to make a whole world map.