I think that was directed at me(?) and that seems a little dismissive? Players are supposed to know how close to a milestone they are? Have you seen how wildly different the chapters in RoT are for example?
No, I have not, moreover, my point was that with exceptions wherein the plot calls for players
not to know how near they are to the next "stage" or completing their current stage, there should be fairly reasonable "tells" that completion is near, or progression is at a certain point. For example, how close are we to Mordor on our trip to destroy the Ring? "How close are we to Mordor?" is broken down into several stages. Have we reached the Prancing Pony? Have we reached Weathertop? Are we in Rivendell? etc.... These are
known points on a map, we know how far A is from B, B is from C, etc... So we know, or in the case of Hobbits who don't know, we learn as we travel how far certain things are from each other. Indeed as LOTR progresses, each of these points becomes further apart, so one can intuit that if it took one day to travel from A to B, and we have been traveling for 2 days and are told we are only half-way there, C must be 4 days away in total.
Figuring out how far you are along in plot completion is exactly the same. In order to imprison the horrible, evil monster, we need the 13 Treasures of Rule. We know they are spread out around the globe, but we have heard that they tend to find their way into the hands of people who would seek to use their power. So, we don't need to look for the Treasures, we need to look for people who are looking for the Treasures. Have we found one? Yes/No. If yes, determine how close they are to finding the Treasure, ie: have they passed Marker A, are they to the POint of No Return, etc...
If chapters are wildly different in terms of determining how close you are to completing the Objective or obtaining the McGuffin that is as I said, a problem with plot progression, because that frankly, shouldn't be happening. Being able to tell how far you are between A and B may be
difficult at times, that's fine, but it shouldn't be unpredictably so. There should always be tells that give players an indication that they are moving forward and in the right direction. Running into more of the Evil Minions is typically a good sign, ever increasing numbers or power levels is a good tell you're approaching the Boss Man. Encountering Strange Effects is a good tell that you're on the right path to finding the Strange Effect McGuffin, these should typically increase in severity and frequency as you approach.
For this I'll refer back to LOTR, the Nine are a perfect example of forward progression. The further you advance towards Objective, the more of the Nine you see at the same time, until of course, you see nine Nine.
If these sort of tells are left out, or the tells do not follow an understandable pattern, plot progression will fail to feel "forward moving". One of the biggest sandtraps of plot-progression-based-advancement are cities. It is easy for players to get bogged down in minutia in a city, from buying things to getting gear to seducing the ladies. This is a common problem when plot-based games fail to include some kind of pressure on the players dallying too long, from risking their attackers catching up, to risking their foes getting too far ahead.
You know why so many TV dramas show people walking from one room to another? Because it makes the plot feel like it's moving forward. You
literally need to have the players keep moving, or it will start bogging down and players will lose the ability to figure out how close they are to their next objective.
It's also worth pointing out that it does also require players looking at things a little differently, instead of looking at "when will I be the next level" as XP promotes, you have to keep them looking at "When will we reach the Objective." Which also helps keep their head in the game. If your players are wondering "when will I level?" there's some kind of communication failure going on because they really should be able to get a feel for it from the understandable plot progression.