ENVIRO-SAFE Pittstop R134A Oil Checker 10 Pack #5030

What if your system has stop leak and/or uv dye. Will this affect the test

The package says to check with the fan on low...is this correct?

Will these testers work for 1234yf system's?

How is it that if the checker shows a trace, the level is ok? Then, what's the point of having 50%marks &100%marks on the checker then?