What to map if I know the turn:lanes value but not where the turn lanes start?

The documentation for turn:lanes says the tag should be applied "from the first indication ... to the junction". What if I know the lane turning indications but not the position along the road where they start (for example, the position where the road widens with the addition of the turn lanes)? Is there a way I can usefully contribute this information to the map? Often, during a trip I'm taking for another reason, I can remember the turning indications but I don't want to go to the extra trouble to try to record the starting position.

So far, I've been adding the turn:lanes tag to the last way before the junction, which may be short or long depending on how the road is currently segmented on OSM (which would depend on what segments needed to be tagged differently for other reasons). In effect, I was thinking of the turn:lanes tag as applying to the end of the way. I believed that even if my practice didn't follow the standard, it was still better than not having the data. And indeed, the main router that I use (OsmAnd) showed the same lane guidance regardless of the length of the last way, as long as it didn't extend beyond the previous junction. But I recently realized that two problems can occur if the way extends beyond the previous junction, which is the case with one way I recently changed (at version 5):

1. If a route includes a turn at the previous junction, OsmAnd shows lane guidance based on my turn:lanes value there even though it was not intended to apply to the previous junction.
2. If another user later splits the way, the editor may copy the tag to both new ways, and the fact that I intended it to apply only to the second way would be forgotten.

So now I'm wondering both how I should fix my previous edits and what I should do in the future if I know the turn indications but not where they start. In some cases, I may be able to get the starting point from imagery, but not in all cases. Would the following be reasonable: add the tag to the last way, but if that way extends beyond the previous junction, split it there before adding the tag to the second part? Then is there a way I can record for other mappers (e.g., in a note tag) the fact that I didn't know the real starting point, so that if they do know it, they know it's safe to overwrite my data? (If it would be better to take this discussion to another place, such as the "tagging" mailing list, I'm open to doing so.)

For that matter, is turn:lanes data on OpenStreetMap valuable enough that I should even bother to collect it and address this question in the first place?