i don't flush at all!
the final 2 weeks the plants pack on the weight so WHY would you want to starve your plants by giving them only water?
in my opinion it's better to gradually weaken the nutrient solution. for example: 2 weeks prior to harvest. fill your res and give it full strength nutes. over the next 2 weeks only add back fresh water to top off your res. in my 20gal rez, in 2 weeks i'll have added back 40 gals of fresh water. by harvest the ppm's of the nutrient solution is very low.
my herb burns great with a nice white ash, i have great resin production and an awesome taste.
peace,