looking forward to this! i've sunk some serious time into this app
while the values you already have for speed might seem high, the npm can be anywhere from two to four times greater than the bpm for a song. the notes for most songs are going to be a fragment of a beat, maybe half a beat, or even a third or fourth of a beat.
some of the songs i've transcribed would sound best at around 600 npm if the smallest unit in the rhythm is a sixteenth. this would equate to 150 bpm. by the same reasoning, the lowest reasonable npm for songs using eighth notes would be around 40, as that would be 20 bpm.
my idea of using italian tempo names in the menu is probably not going to work out, since npm doesn't account for meter and division of the beats. 260 npm would be Vivace (130 bpm) if ♪eighths♪ are the shortest value, but Adagio (65 bpm) if ♬sixteenths♬ are the shortest value. i don't think there will be an easy way to incorporate it