For Cattleyas, I use as high as 750 microSiemens, or uS ( the u should be a Greek letter mu but not on my keyboard ). NOT mS which is milliSiemens, 1000 times greater.
That is when in growth with good light, usually Summer. When resting , nothing or just plain water - if food rain, probably down as low as 100 uS, or from RO, as low as 20 uS. If growing in winter, as they do under lights, probably 600 is good.