How does the mixing adjustment work ? Can I adjust it to add 1 drop per litre of water ?

Yes, for example if we called one drop for instance one milli litre (ml).

Than with the cap setting on slow ( 1000:1) for every 1000 ml of water ( one litre) trough the hose it follows that 1 ml of product would be dispensed.

The fertiliser to water ratio is set by the adjustment on the top of the cap (SLOW, 1, 2, FAST).

On the SLOW setting the dosage rate for the 3.2  tank would be 3200 litres of water to 1 litre of product.