I don't advise the use of the apps. A camera uses reflected light. A par and lux or footcandle meter measures incedent light.I've understood as you do and wonder myself about it and these free apps people using to place corals?
Plus cameras have color issues like limiters and balanceers used to make a nice picture.
And it's a lot harder to use.
I use my meter to establish a range in my tank. If it were a par meter I would do the same. Super high at the top and super low at the bottom because it's deep. So now I have a bit better idea of what goes where.
That said my first really sucessful acro is at the same distance from the light that my colony of Xenia are at the very top of the tank.
I have have three ai sol at at 65% eight inches from the corals. No matter how you measure it. That's a lot of light.