There are several things to consider when converting times from one frame rate to another.
Movie conversion
A frame rate conversion basically consists of converting your subtitles in the same way the NTSC movie was transcoded to PAL.
The classical method is by downsampling or oversampling, this is, adding or removing frames.
For example, when converting from NTSC 30 to PAL 25, we need to remove 5 frames in each second to make the frame rate change from 30 fps to 25 fps.
However, movies not always lose frames during the conversion.
For example, a 24 fps movie converted to 23.976 fps keeps the same amount of frames, it just has a different playback speed.
In this case you do not need to convert your subtitles by adding or removing frames, you just need to change their playback speed.
Garbage
Some movies may have garbage at the beginning, for example, the movie has some black frames or a counter at the beginning. This is common in movies files that come from a tape.
If you have two movies (a NTSC and a PAL ones) it is very probable that the garbage is different in both movies, the garbage may have different length.
In this case, after applying a NTSC to PAL conversion, the subtitles still may have a delay. But it is because of the garbage.
A simple time offset may fix the problem.
Editions
Make sure your NTSC and PAL movies have the same edition.
If one movie has additional or removed scenes, the conversion will never match.
Frames
Lemony Pro work internally with frames, not timecode.
Timecode is just a way to name frames.
For example, frame 60 is labeled as 00:00:02:00 in NTSC, and 00:00:02:10 in PAL. Same frame, but different name depending on the frame rate.
When you change the frame rate of your project, you are just changing the way frames are labeled.
To actually convert the times, changing the frame rate is not enough. The frame values must be converted too.
If you just change the frame rate, frame 00:00:02:00 in NTSC will become 00:00:02:10 in PAL, and not 00:00:02:00 PAL as it should be.
So additionally we need to multiply the frame value by a factor, in this case by 0.8333 (25 ÷ 30).
Frame 60 × 0.8333 = 50, which is displayed as 00:00:02:00 in PAL.
With the duration of the subtitle happens the same. It has to be factorized in order to match the new frame rate.
A 3-second subtitle (90 frames NTSC) must be multiplied by 0.8333 to become 75 frames (3 seconds PAL).
Constant delay
If your subtitles have a delay, but it is constant (not accumulative) you do not need to convert the times. A simple offset should fix the problem.
Burned-in timecode
Sometimes the workcopies of the movies come with burned-in timecode. Make sure the movie's and timecode's frame rates are the same.
For example, it is possible that you get a 29.97 DF movie with 30 NDF timecode. This may be misleading, and may make you to to a wrong conversion.
Separation
The time conversion may affect the separation between subtitles.
If your original subtitles had a 4-frame separation, you may end up with a separation of 3 or 5 frames.
In the same way, subtitles without a separation may end up with an overlap.
Some file formats, such as Blu-ray BDN Text, require a minimum separation between subtitles.
So, after a time conversion your BDN subtitles may become invalid.
Closed captions
It may be difficult to convert closed captions to a lower frame rate. For example, from 29.97 to 23.976.
This is because captions require a specific amount of frames to be streamed.
If a caption requires 60 frames to be generated, and the conversion reduces its duration to 50 frames, the caption may get truncated or never displayed.
One solution is to remove double commands to make the build-time shorter. For example, convert all {RCL}{RCL} to {RCL}.
You can also remove the italics to make the build-time shorter.
Check
Always check your converted subtitles against your final movie.
If you do not have the final movie, then you are doing a blind conversion.
The risk with blind conversions is that you do not know if the resulting times will match.