It depends on how much water is in that 400' well. If the water level is close to the surface, you may have 600 gallons of water standing in the well to chlorinate. If the water level is at 380', then you only have about 30 gallons of water in the well to chlorinate. It shouldn’t flow away in the well if you are recirculating the same water back down the well with the hose. If there are 600 gallons in the well, you may have to circulate for an hour with a 10 GPM pump before you smell anything but, you need to have enough to smell in the first place. However, chlorine that is too strong can eat away metal parts in the system, so don’t put more than you need.