I am not certain how Rhumb Lines function. I haven't had the opportunity to look it up, yet. My world map is roughly 3,000 miles x 3,000 miles (same as the pixel count). Although, that might change once I start filling the land mass with junk. Most of the terrain features are representative, and the scale and details will be laid out in regional maps (assuming I don't die of old age first). It stretches from just below the equator to just above the arctic circle. I don't believe my map is too large for the lines you posted. Please correct me if I am wrong.
I did read that the center rose was placed at the port of call or major port or home city or what-have-you. However, the largest city on my map lies near the center, and I thought I might say that the Rhumb Lines were used for aerial travel as well. I am not certain how feasible that is, and I don't know what purpose the other compasses/roses would serve. As it stands now the Rhumb Lines basically encircle the mainland and pass between smaller outer islands, for the most part. I was thinking the system could have been invented by an explorer who first circumnavigated the continent, or something. I am quite fond of the way it looks, and I don't want to change it. I want it to make some sort of sense, however.
I have decided to keep your image as the background instead of water. The lower lines set depth, and I wanted to go with a brown motif for all of the terrain features, rivers, lakes, etc., except for flora. If I didn't I would still be collecting map elements this time next year. By keeping all of the regional elements the same basic brown motif I can get the world map situated faster and then concentrate on regional maps and their individual palettes. And since it's not a realistic portrayal of the land I can get away with using your Rhumb Lines as my backdrop instead of actual water.
Abathas WIP.jpg