Is there any way to adjust the relative duration of slashed grace notes, as opposed to unslashed ones?
If there were, it would be in Playback options>Timing, and there is a setting for default grace note length (which should apply to slashed grace note only, as an unslashed note “robs” half the value of next note, IIRC)
Thanks. Pardon my obtuseness, but I don’t comprehend why the default grave note length setting refers to fraction of a quarter note at 120 BPM. Surely the fraction - not an absolute duration - applies at any tempo? And if this is the case, what if my tempo is different?
The value is scaled according to the tempo, but it is expressed as an absolute duration in real time rather than as a musical duration.
Huh? 1/6, for example, as shown in my Doricio, is a fraction, not an absolute value. I really don’t follow this.
So how am I supposed to work out what it is at say 48 BPM?
[Worse than this, it appears to make not a jot of difference with NotePerformer. I must be missing something fundamental.]
So a quarter note at 120bpm is half a second in duration. 1/6 of 0.5 is 0.083. So at 120 beats per minute, the acciaccatura will be 0.083 seconds in duration.
If the tempo were 60bpm instead, then a quarter note is twice as long, and so the acciaccatura will be twice as long too, i.e. 0.166 seconds.
At the moment, I don’t think there’s a limit on this, so if you go down to 30bpm, the acciaccatura will be twice as long again, i.e. 0.32 seconds.
OK. So why is the BPM fixed at 120? Why can’t you edit that? What is the rationale? Why can’t we specify 1/8 of a quarter note, without any reference to BPM? This is what I still don’t get.
If you wonder why I am being so dogged about this. it’s that I am making an urtext replica of the Haydn String Quartets, and I want it to be a very visually precise replica of the 1803 edition, wherein are used slashed graces as well as unslashed graces, but the slashed graces are most often, but not always, interpreted as appogiaturas. So the playback renders all the slashed graces as rapid acc.s and in this particular, admittedly very unusual and rare use case, it makes the playback uncomfortable, and wrong, for this music. I’m probably the only person on Earth with this requirement. 
The fact of it not making any difference to NotePerformer on my system perhaps I should create a new thread?
There’s an explanation for the similar formulation of duration of arpeggio signs here - basically, for notes that are usually played quickly, it makes more sense to give them a fixed speed rather than a relative one because you are therefore less likely to run into situations where they play back much faster/slower than desired. I.e. most of the time, slashed grace notes are played with a similar duration regardless of both the tempo and the duration of the following note.
@Andro, I’m an avid string quartet player in my spare time, and a great lover of Haydn. I always like to delve into original sources, so I’m interested in your work. How is your version different from e.g. the Henle edition? Is this a commercial project? Will it be published?