How to calibrate floating hydrometer ??

if i have RO water where SG must = 1.000
so if hydometer reads = 1.001
so i will subtract 0.001 for my salty sample ???????
(all at same temperature )

 

you need to get a calibrated refractometer, pinpoint sells a 1.026 solution that you can use.....if you want to use the hydro just use a freshly calibrated refract to test the same water to remember how far the hydro was off(if it is)

 

also never calibrate using ro water

 

A floating hydrometer should NEVER need calibration. If you have one that is off it has to be a pretty cheap floating hydrometer.
I've already posted the correct way to use one in your other thread.
The only things I didn't write were that of course it needs to be kept clean because any deposits on the glass can affect the results. Also, you do have to be careful as just in moving it around, sometimes just light contact with something solid will bread it.

 

Unfortunately, not all hydrometers are suitably accurate.In addition to such calibration issues, however, is the significant problem of temperature effects on hydrometers.The two types of hydrometers are different in some important aspects. The most important of these differences is that swing arm hydrometers do not usually require any corrections for temperature effects over the range normally used by aquarists, while standard hydrometers often do.Randy Holmes Farley

Heres a good link about Hydrometers and salinity.

Temperature Corrections for Hydrometers by Randy Holmes-Farley - Reefkeeping.com