I used to seriously lift but now am just getting back into it for sports training, but may try your method someday.
I tried an app today (Android) and it worked OK, but I kept the phone in an armband I use for running, and it's not the most convenient to take off and put on, so maybe like a waist strap ? or something would be better. I am leery of not having the phone attached to me in some way. I just know I (or someone else) would drop a weight on it if not or I would forget it somewhere.