The rules, and the novels, mention that the Phlogiston is divided into two general areas: areas with no orderly pattern to the movement of the Phlogiston, and "rivers". The vast majority of the Phlogiston moves randomly, which is almost no help at all when sailing. However between many spheres there are areas where the Phlogiston flows very rapidly, and all in one direction. These "rivers" allow very rapid (and fairly safe) travel between spheres. I drew a map of the relative positions of the spheres, and then drew the various rivers. The rules gave some information as to what kind of rivers exist, and even what rivers exist between the most famous of their worlds[1].
Rivers flow either one way, or both ways. Rivers which flow both directions are fairly uncommon. It is impossible to travel against the current. For example: there is a river flowing from Krynnspace to Realmspace. This makes it very easy to travel from Krynnspace to Realmspace, but impossible to travel from Realmspace to Krynnspace.
Traveling in a river does not require that the navigator know Phlogiston Navigation[2]; Phlogiston Navigation does make life much easier though.
In a river the base travel time is 30 days. The minimum travel time is 5 days. There is no possibility of getting lost.
If a Phlogiston Navigation check is made successfully, subtract five days for every point by which the navigator made the check (to the minimum time). If the navigator rolls a 1 then the travel time is reduced to 5 days.
Example: Gillian Starblade must make a 16 or below to make her Phlogiston Navigation check. She rolls a 13. Since this is 3 less than what she needed the travel time is reduced by 15 days.
The drawback is that if the check fails, then the travel time is increased by five days for every point by which the check failed. A 20 results in the travel time increasing to 60 days.
Traveling without a river is possible, but slower, and there is the possibility of getting lost. The navigator must have the Phlogiston Navigation proficiency or else the ship will be totally lost, and probably never make it to any sphere.
The base travel time for open travel is 70 days, minimum travel time is 20 days. It is impossible to get anywhere without a Phlogiston Navigation check.
If the check is successful then, as it is in a river, the travel time is reduced by 5 days for every point by which the check is made. If a 1 is rolled then the travel time is reduced to 20 days.
If the check is unsuccessful then two things happen. The most obvious thing is that the trip takes 5 days longer for every point by which the check fails.
If a 20 is rolled, then the trip takes 130 days. In addition, the ship does not arrive at the destination the captain had in mind. The DM finds an appropriate sphere (i.e.: a sphere near the origin sphere), and the ship is there.
My major break with the official TSR rules is that according to those rules it takes from 10 to 100 days for a ship to travel to absolutely any sphere[3]. I consider this to be totally absurd. I also see nothing wrong with directly going against the stated rules, but one should be aware of the rules one is going to break. :)
I can understand TSR's reluctance to make anything more firm, since that would entail an official TSR map of the spheres. They obviously intend DMs to invent various spheres of their own, and place them appropriately.
I made my own map of the spheres and keep it handy. When the PCs want to travel without a river, I use a bit of judgment. Spheres which are near to their current sphere can be accessed through the open phlogiston.
The drawback is that the word "near" is vague. The phrase "adjacent spheres" comes to mind, but the spheres on my map aren't laid out in a neat geometric pattern, so it is more of a judgment call. Generally it should be fairly obvious to a DM which spheres are near enough to allow open travel. There is a great deal of leeway here since the flow is chaotic, and could quite easily be denser in some areas allowing travel to spheres further than it would appear possible.
I can think of no simpler method than good judgment on the DM's part.
