bowlesj
does your vocalist have daw software ?
if so just give the vocalist a bed traks stereo mix from biab in different keys ? for practice , and to see what works .
then youll get feedback from vocallist which one is preferred ,
and which one the vocallist feels more comfortable with.
thats what i used to do.
also most vocallists know anyway in advance what they are
comfortable with, thus this should narrow down choices.
ive done the above on gear from 2 inch MT machines down to portastudios. lol.

have a merry.
muso.

my songs....mixed for good earbuds...(fyi..my vocs on all songs..)
https://soundcloud.com/alfsongs
(90 songs created useing bb/rb.)