Many knowledgeable minds have already contributed, with medication available within UK (non-prescription as in the above) the safety margin is very generous comparatively to prescription only dispense. Flock dosage is a generic average value indicated (as one bird may drink more than others and vice versa). I'll refer to a commonly used term 'preventative' (a word worthy of an academic deliberation for sure), as in your case above. The question arises will 12/13 birds drink a litre of solution? (especially in the winter, when water intake is at the lowest consumption ratio). A point to cater for is, this is a medication serving not water to quench their thirst, hence when serving quantified solution(s) it is advisable to use a galvanised ceramic pot (which holds approx 500ml), sufficient for the numbers per section as you have respectfully outlined above. Simply, if they require more, top-up with a needful measure, this mode of delivery will help to eliminate wastage (just make sure the birds are shown less water at other times when medicating). Now moving ahead with number of days of dispensation (mindfulness of marketing and commercialism will serve the best antidote to means of feasibility). Any element of antibiotic(s) will not serve a preventative measure of any kind but rather curative in the presence of an identifier (targeted bacteria to be treated). Without correct testing/informed evaluation its a shot in the dark but equally due to the nature of the keep (birds in your loft) over the course of a given season/year, there could be a element build up of certain common bacteria found generally amongst the birds/lofts, the very reason it shows signs of improvement amongst the treated (if there is an issue of bacteria build-up as mentioned not every loft will have that). As for PMV administration, it really depends on the level of immunity as per the product being used, this is very different to the so called 'preventive' approach with use of antibiotics, the former truly works to build antibodies against the variant its been administered for and as for the latter over usage will result in building resilience to antibacterial drugs, which usually results in creation of a 'superbug' a simple example would be Adeno-Coli etc. I trust the above will aid the deliverable measure adequately.
Plane
Describe directions for people who are getting to the venue by plane.
Bus
Describe directions for people who are getting to the venue by bus.