The lightmeter apps are accurate. As long as you know what you are doing. If you use it in reflected light mode make sure you point the camera on the phone at something 50% gray. I like to use the palm of my hand (I have dark skin). If you have lighter skin take a reading and add+1 stop. The more accurate why is to do incident light mode. You will have to buy a small attachment that goes over the phone lens to get a accurate reading. If you want a rough estimate switch to incident mode and point the lens at the sky. Make sure the sky is uniform (this is important). Either blue sky or uniform cloud.
Whatever method read the light at ISO 100. The EV value you get can then be converted to lux or FC. Heres a good link
Conversion Chart
I've used this method to shoot slide film on old cameras. It works. even on my crap phone. Should be no problem for what you are trying to do
Threaded Mode
