Counting bunches per vine
About a month before harvest many vintners count bunches per vine, this more or less lets you know what weight of grapes you will harvest. You need to know actual number of vines (see here) and can approximate a bunch weight based on general average numbers in the UK or measure your own. Many people do measure bunch weight to get an approx historical bunch weight that is specific to your site, this will mean your yield predictions should get better year on year.
Yield (kg/acre) = actual number of vines per acre * no. bunches per vine * bunch weight (kg)
Sectormentor app makes it very easy to count bunches per vine as you scan the tag at each sample site using your phone. You then enter the number of bunches you count on each vine for the next 10-20 vines (depends on your sampling system). Sync when back in the office and you will see the average bunch count per block/variety straight away.