One thing people seem to routinely forget is it that in order to get a flare-free spot reading, you need to properly shade a spot meter just like a camera lens. I bought collapsible rubber hoods for my Pentax spot meters. You'll have an even bigger issue with lesser meter lenses which are not as well coated as the Pentax.
I'm rather skeptical how accurate as cell phone app would be, for several reasons, including the foregoing.
As it is, I have a problem with tiny meters. I either have to carry around reading glasses, or dig a magnifying glass out of my pocket. And it's really hard to beat the intuitive manual dial on the Pentax spot meter. I much preferred it to the pushbutton method of the Minolta F, even though they read identically.