Does make any difference to have a beam fire control of 6400km of range or 10000(or more) if we speak about PD against missiles? As far as I remember laser in PD fires in the last 10000km before the missile hits, so, is ineffective a laser that fires at 6400km?
A BFC with less than 10,000 km range will not be able to fire at 10,000 km, thus no PD fire. Additionally, the longer the BFC range is, the better the accuracy at 10,000 km will be, scaling as (10,000 km / BFC range in km). In fact, a BFC with exactly 10,000 km range should have 0% accuracy for PD fire.
As long as the weapon has at least 10,000 km range (which I think all of them do unless you're screwing around with the DB), weapon range will not affect the accuracy for PD fire, only the BFC matters. Though particularly with lasers you may want added range to deal with stand-off/laser warheads.