Adrian,
The answer is quite simple, it is of course,
q = qe +qse + qmc
Where
qfe = 61 * fdcfe/pqfe
qse = 92 * fdcse/pqse
qmc =212 * fdcmc/pqmc
q is the quantity
fdc is the family's daily consumption
pq is the output per tuber (averaged if more than one variety of a type is grown)
the subscripts fe, se and mc refer to 1st early, 2nd early and maincrop respectively.
Notes.
1. the number at the start of each expression is the number of days consumption of that type of tuber, this should:
a. be increased by 1 in a leap year
b. be reduced by the number of days holiday taken, unless you are self catering and taking your potatoes from home
2. Where a salad variety is grown, a similar expression can be used and the relevant quantity of the variety/varieties being replaced, reduced
3. If you believe that you'll believe anything :D.
What you don't say Adrian is how much potato your family eats that is the main factor as there's no point in growing what you don't eat.
Rough guides for the "output per tuber" (dependant on such factors as, variety, soil condition, weather, time planted, time lifted, blight, etc, etc) and vary widely
1st early 1 - 1.5lbs
2nd early 1.5 - 2.5lbs
Maincrop 2 - 3lbs
Phil