Writing down exact lengths this is what a piece of my melody (4/8) looks like:
I assume this is a text book case of it being better to use a grace note like this:
I'm having a bit of a hard time on deciding when exactly to use grace notes. For instance, this:
My intuition is to leave it as it is, since it occurs in between beats (as it should) But then it seems odd to have fixed-length notation in some instances and grace notes in others for the same kind of embellishments in the same score.
Grace notes and phrases do not as a rule carry harmonic/melodic/rhythmic meaning. Any accented note that is off-beat is not a grace note. Notes that are on-beat may be appoggiaturas (a particular kind of grace note asking for harmonic resolution, not stricken through and usually written with "at least" its nominal duration): they are a bit of the odd man out since they eat into the note value of the following rather than the preceding note, serve harmonic function, and it's actually the following, usually "in-harmony" main note that is unaccented and may even have a fuzzy/shifted starting point.
Appoggiaturas are not much in fashion these days. Their main place is in solo pieces or voices or chamber music where the execution typically relies on single players.
So as a rule of thumb (and putting appoggiaturas aside): if the timing of the following note is more important (and likely accented) than the timing of your current note, a grace note might be appropriate. If the pitch of the following note is the same, a grace note is almost never used. In a similar vein, if a particular articulation is prescribed (other than just using a legato slur, typically including the following note), grace notes are more often than not replaced with regular notes.
There are no completely reliable hard rules like in many musical issues.
Grace notes are usually used as Acciaccaturas. So, you'd use them when the notes are not melodically and harmonically essential. They are ornaments, that add a little flavour to your melody. Basically, if you remove them from the melody, the sum of it would be the same.
Also, this:
Is not so easily readable by someone not so experienced. I'd suggest to use something less confusing, unless you are aiming this to be read by someone on a high level of sight reading.
