5 grams of Citric Acid to 500 ml of water. The 500 ml comes out to close to a pint of water. Use a decent scale that can weigh in metric. I weighed out the Citric Acid by placing 1/4 teaspoons at a time on the scale and after putting 5 of the 1/4 teaspoons on the scale it came out to 5 grams. So, 1 and 1/4 teaspoons.
The Citric Acid that I found in a local supermarket is not a powder. It is finely ground crystals. I recommend that any new batch of Citric Acid be weighed out since the volume of 1 and 1/4 teaspoon could weigh differently depending on how finely ground it is. Keep the amount in the measuring spoon as level as possible to maintain a consistent method.
If the Bud Rot is already showing then the rotted area should be removed before spraying. If it was me I would then spray the entire plant even the remaining buds. The citric acid would create a hostile environment that reduces the possibility of any surviving mold or mildew from being able to start up again.