That is exactly the pointy that I’ve been trying to make - how to sensibly achieve a target level is different from one audio track to another.
For the three screenshot samples that I’ve posted, if someone said they want to fade in with a 50% or -6dB target level, I’ve no idea what they mean!!!
I can assume that with Koz’s piano sample that they mean they want the final note to have a peak at -6dB
I can assume that with the “pop music” track that either they want the final quiet section (the last 5% of the track) to have a peak of -6dB, OR they want the end of the final loud section to be -6dB.
I can assume that with the “tone + silence” sample that they want the end of the tone to be -6 dB, OR they have lost their marbles, OR they are imagining that the amplitude of the tone continues to the end of the selection and they want that imaginary line to be -6 dB at the end of the track.
Alternatively I could assume that in each case they want the final non-zero peak to be -6 dB
OR
I could assume that they want the average peak level of the entire track to be -6dB
OR
I could assume that they want the target level to be -6dB based on the highest peak in the selection
OR
I could assume that they want the target level to be -6dB based on the highest peak in the last “x”% of the selection (with a control to set “x”%)
OR
I could assume that they want the final peak to be -6dB unless that causes clipping anywhere in the track, in which case the fade should be as steep as possible without causing clipping
OR
I could assume that they will only use this effect on constant amplitude tones
OR
I could assume that what they really mean is that they want a target amplification amount, but they don’t realise that is what they want
OR
I could assume that they have been working on a few highly compressed dance tracks that have a substantially constant amplitude and want to set a target for the mean peak amplitude.
OR
I could assume that they want to fade to a specific loudness
OR
They could mean that they want a plug-in that automatically works out what they mean and does it (could require dark magic or cerebral implants 
)
If I try to implement any of these, it is likely to be the wrong solution in the majority of cases (except for the “magical” one).
OR
I could make a plug-in that has all of these options and requires a huge screen resolution to accommodate all of the options and a 400 page manual explaining them all.
and I would like a “Professional Audio Filter” Professional Audio Filter
