The FAA has assigned conversion values which do not have a very good mathematical correlation to each other. I know these off the top of my head...
1SM / 5000
3/4 SM / 4000
1/2 SM / 2400
1/4 SM / 1600
You can use reported vis instead of RVR for CAT-I and non-prec approaches only. CAT-II/III require RVR.