Iggy, I think perhaps what you need is a little knowledge of projections and distortion. What are the dimensions of your current map? In many ways the easiest projection to work in is the equirectangular, which is just a regular grid of squares with latitude and longitude plotted on them like a graph. This projection uses a 2:1 image, for example 6000 x 3000 pixels, and it's perfect for using as an overlay in Google Earth so you can see how things "really" look on your spherical world.

The next step is to read up about and play with the distortion. Basically, there is no perfect projection of a sphere - all projections distort something, and in fact most projections only preserve one or at most two aspects of the following: shape, area, distance and direction. Some preserve none of these aspects, instead choosing to compromise.

What you need to know about equirectangular maps is that they preserve distance along the equator and along all meridians (north-south), but east-west distances away from the equator are progressively more and more stretched, until the poles are represented by lines the entire width of the map. You can clearly see how this works if you overlay your map on Google Earth.

The approach I have taken with this in my world maps is to start off with an equirectangular map, then reproject it using G.Projector to do both poles, as well as any other region I want an exact shape with. Specifically, if you reproject into equirectangular oblique centred on your area of interest, then do your design, and finally reproject back to equirectangular and update your original map, you should end up with a good base map. G.Projector is a bit fiddle in this regard, but it does work.

The final step of course is to choose whatever projection you like best to present your world map in. It's important to remember that no one projection will do everything you want it to. Also, you should realise that large scales (local) are inherently less distorted than small scales (regional and world). If you want to do measurements, it may well be better to make several large scale maps of important areas.