The only flush i do in coco is the final one at week 9,or if i have a problem,during the grow other than that there is no need to flush.
But saying that i do only feed via root's,with auto pot's so after the first few week of feeding the coco when the clones hit it,after that the plants,roots hit the bottom of the auto pots they feed by taking up nutrients via roots from trays with a air ring in the bottom.The air ring is key to not getting salt build up's and keeping the nutrient film in the trays,with a good supply of air.
After the first 7/12 days the plants then receive no more feed via the coco,its just to support the plant's,the roots do grow at a fast rate in it,but no feeding is done through it.At the end of the grow when the plants are cut down to hang the auto pot should be full of root's because a air ring is used the root's are lovely white color ,right up to the end.