The future of drones in agriculture is bright, but UASs must be used with human beings and the ecological system in mind.
To calculate the spray rate, you will need to talk to an agronomical engineer. Drones have limited payload (=weight they can carry), restrained autonomy in terms of power (it depends on the payload and other constraints) and the spraying must entail the strictly necessary concentration of chemicals for the given crop bearing in mind the living organisms.
Depending on the country, you may have to comply with legislation and guidelines.