you wouldn't calculate x to make the swap have a pv of zero at inception, it would be the fixed rate, i.e., the coupon on the fixed leg, or the forward swap rate. say i receive floating and pay fixed,

floating leg: + LIBOR + x

fixed: -k

then i will calculate k to ensure that the PV

sum_{i} P(0,t_i) (t_i-t_{i-1}) (L(t_{i-1},t_i)+x-k)

is equal to zero

x is usually the spread or the margin added onto LIBOR, it would be specified in the contract a priori to any calculation being completed.

read brigo's book on interest rate models