Why a set always runs longer than the songs
Add up the tracks and you get the music. What you play is the music plus everything between it: tuning down for the one song in DADGAD, saying who wrote the next one, waiting for the room to stop clapping, finding the capo. Thirty seconds a song is a conservative guess, and on a twenty-song set that is nine and a half extra minutes — the difference between finishing on time and being asked to.
The gap goes between songs, so a set of twenty has nineteen of them and none after the last. That sounds like pedantry until you notice it is the direction that makes a calculator over-promise.
Write lengths as 3:45
One form, on purpose. A bare 4 at the end of a line is far more often the end of a title — Interlude 2, Take 4 — than it is four minutes, and a calculator that ate it would report a shorter set than the one you are about to play. Anything without a length counts as the default you picked above, and the table marks it as assumed so the guess is visible rather than hidden in a total.
The finish time is rounded up
A slot is a promise to somebody holding the keys to the venue, so a set that runs 44:20 is a 45-minute set here. If the number matters — a curfew, a support slot, a wedding — being wrong late is cheaper than being wrong early.
Nothing leaves your browser
The arithmetic happens on your own machine. Nothing is uploaded, nothing is stored, no account — which is worth saying about a document that names what you can play and when you are on stage.
When the list stops being a list
A set typed into a box is fine once. What it cannot do is be the thing you read from: reorder with a finger during soundcheck, hold the key each song sits in for your voice, be on the phone in your pocket as well as the tablet on the stand, and open with no signal in a basement.
That is what Strumfolio does with a set — songbooks that are one set each, split into sections, reordered by dragging, and readable offline once opened. If your songs are text files right now, the ChordPro converter is the way in, and the transposer is one page over for the ones that sit wrong.