The original recording key is not automatically the best key for every singer. A practical choice considers the lowest and highest notes as well as where most of the melody sits.
Find the difficult points
Sing through the most demanding chorus and note where the voice tightens. Then check the lowest phrases. A useful key must work at both ends.
Tessitura matters
Touching one high note once is different from singing many lines near your limit. If the song lives too high, lowering one or two semitones can improve tuning, tone and endurance.
Move in small steps
Transpose by one semitone, sing the difficult passage and repeat until the voice feels free. Then perform the whole song so the lower range is still comfortable.
Consider the instrument
A singer-friendly key may create awkward guitar shapes. A capo lets you preserve easy shapes while the sounding key suits the voice.
Save the rehearsal version
Record the final key and capo position on the chord sheet so every musician prepares the same arrangement.
One more check before deciding
Record the same chorus in two nearby keys and listen back the next day. Compare pitch stability, consonant clarity and whether the singer can shape dynamics instead of merely reaching the notes. If a key only works at full effort, it is usually not the most reliable rehearsal choice.