I use that and the Pentax Digital Spot meter in addition to meters in the cameras, but if I have a meterless camera I use hte Sekonic or want to get more shadow detail I use the Spot Meter.
So, will my photo be all slimy if it is under exposed?
On the question of smartphone apps--accuracy of the meter has more to do with the camera hardware than the app. Most of those apps just turn the camera on, and read out the auto exposure settings. So if it sees the camera set at f5.6 and 1/1000, and you've set the app for f16, it will just do the adjustment for you and tell you that 1/125 is the correct shutter speed. It sounds like the app that @wiltw is using is allows you to add an offset to that conversion. Its possible some apps could build their own metering system by turning off auto exposure setting it to a fixed exposure, then pulling a camera image and doing averaging on the pixels, but those are still going to be dependant on the hardware.
One thing I wish that apps would have is the ability to display exposure using different aperture or shutter scales. Of course most/all handheld meeter only support a single scale as well, but an app would have a very very easy time displaying with older scales. I have a fair number of LF lenses in older shutters with both aperture and shutter speed scales that don't match the common 5.6/9/11/16/22 or 15/30/60/125/250/500 modern scales.
f/11 +0.1EV to f/11 +0.7EV at 1:30pm on a birght cloudless sky at 38 degrees latitude. So here right now, Sunny 16 would be underexposed by -0.3EV to -0.9EV! Color transparency would do fine, but color neg would be getting into the possibility of 'muddy color' in the shadows.
I detected readings between f/8 +0.6EV to f/11 +0.9EV at 2:30pm on a birght cloudless sky at 38 degrees latitude. Deviation from Sunny 16 got even greater, by -1.4EV at one side, but better at -0.1EV at the other extreme.
I just did another set of readings with Minolta Autometer Vf incident meter, on a bright sunny cloudless sky at 1:30pm at Latitude 38
Readings varied by angle, with meter held perpendicular to the ground as if 'aimed at the camera lens'... f/8 +0.8EV to f/11 +0.7EV
This may sound crazy, but I use "sunny 16" to calibrate my expensive digital light meter, I point the lumisphere at the sun on a cloudless sunny day with the i s.o set at 100, and the shutter speed at 1/125th, it should give a reading of f 16, if not I adjust it until it does.
A couple days ago my sunny 16 went sunny 11. It's back to sunny 16 today. I don't know what happened. There was no cloud or anything thing to cover the sun that day.