Anybody have tips or tricks for finding the optimum key to fit a specific singing range?

Singing is not my strength, and so I have spent quite a bit of time trying to figure out where my natural range lies, and whether that means all my songs should be in the same key. Although I haven't come to a firm conclusion yet, my "working conclusion" (subject to change based on new information) is this:

1) every singer should know his/her range (lowest, highest and most comfortable notes). In my case, all three of those are B flat. But when I try to put songs in the KEY of B flat, it doesn't always work

2) rather I have found that if I transpose the MELODY until all the notes are centered around B flat (even though the sharps and flats may put it in a different key) I can usually sing it comfortably.

3) I have also determined that the mode of the scale affects which key will put the melody in the middle of my natural range.

But I still haven't come up with an empirical formula that identifies a key based on information other than trial and error. Anybody have thoughts to add?

https://www.dropbox.com/sh/ey71h8ep047758h/AAC12jNM0Hz9Q8zsh5iDeqNHa?dl=0