Sorry, it's been a while since I played SH4, so I fired up the game and found it.
Go to the "Nautical Chart" command, and there is a button for "maximum range". That will give you your max range at current speed.
However, that is in RFB 1.52. I seem to remember that you can do the same in TMO as well. I don't know about stock. I haven't even tried stock, I jumped right in with the super mods. With my experience in SH3, I knew stock would be a complete bore.
Hope that helps.