Do I remember well that the grace notes have a duration of 1/32?
Grace notes take 1/32 from the next non-grace object that has duration (note/rest/chord/restchord). There are limits (16?) on how many grace notes can appear uninterrupted.
I can't speak for your memory.
It's better that way, trust me. ;-)
Thank you.
A single grace note has a 1/32 duration, and dotting and tripletising don't affect that. But you can lengthen the duration by tying them together and hiding all but the last one.
See also: Il trillo del diavolo (https://forum.noteworthycomposer.com/?topic=6771.msg46410)