I think Andy has one of those basic water softeners that use a di vessel to store the salt in.
The sequence for your setup is;
water softener, containing salt. Water passes through the salt and the calcium and magnesium ions are removed and replaced with sodium ions. (The water will still be 296 at the tap, but salt is kinder to the membranes than calcium and/or magnesium.)
then r/o unit comprising of,
sediment filter,
carbon block filter/filters - there maybe 2 of them - a GAC filter and then a carbon block,
4040 housing with membrane,
then a di vessel on the pure line to either van mounted tank or IBC holding tank. This di vessel will be filled with resin to 'polish' off the remaining tds to zero.
On the 4040 housing you will have the water inlet from your filters, then a waste and pure outlets on the other end if its a champ housing. The pure will go to the di and the waste will go to the drain/soakaway.
On the waste will be a valve of some sort that will allow you to restrict the amount of waste going to the drain. We have ours closed to a point where its producing about 1 litre of pure to 1 litre of waste. This is giving us a 98 - 99%% rejection rate ( eg., 125 ppm (TDS) in, 2ppm out.)
To flush, we open this valve fully allowing the water to wash over the top of the membrane and flush any deposits way. To stop flushing, close the valve until you have you waste to pure ratio at about 1 to 1.
If you can't achieve a pure ppm of around 5 to 6 with a tap tds of 296, then you may need to replace your membrane.
We have 40 psi at the tap, use an HF5 membrane and do not have a booster pump.
NB!
Do not flush your membrane with tap water as that contains chlorine - chlorine is a membrane destroyer. Only flush with water that has been through a reqularily replaced carbon block filter.