Yes, Bonide Rose RX Systemic Drench Concentrate can be used for both a preventative and curative application. The mixing rate is 1-2 fl oz per gallon of water. You will apply every 7 days to control active disease and every 14 days for preventative treatments.
From page 4 of the product label for Bonide Rose RX Systemic Drench Concentrate: FLOWER BEDS: 2 fl oz (4 tbsp) per 2 gallons of water to treat 12 sq ft. You are not just treating the plant itself but are also applying to the surrounding area of the plant so that it will be absorbed by the roots. The best way to determine this would be to measure the square footage of the area you need to treat to determine the smaller amount needed and divide accordingly. IE if only 6 square feet is needed to treat you would use 1 fl oz in 1 gallon to treat this 6 sqft area.