From the area 51 site
We recommend 30" above seedlings in 130w mode, 20"-30" above vegging plants in 130w mode, and then 20"-24" above flowering plants in 190w mode.
If your grow space is limited, you could always run the lamp seed to harvest on 130w mode. 14" to 18" above the plant tops is more than enough intensity and coverage for flowering plants in a full 24"x36" grow tent. You could use this lamp in as small as a 24"x24" grow area with as little as 48" of height, based on 14" from the canopy and 18" tall plants and 12" tall growing container.
We suggest to look at our Umole charts on the Specs and Datasheets page, and position your XGS-190 above your plants according to those charts.
Seedlings need 200-400 Umol'/s, vegging plants need 300-500 Umol'/s and flowering plants need 500-800 Umol'/s in its core area and 150+ on the edges. Anything more than 800-1000 Umol'/s is not necessary as it will not add to yield, size, density or potency. Some plants will show signs of damage even at some levels below 800 Umol'/s.