I don't think it is that clear and dry. I rule it on a spell by spell nature.
Blindness has a 30ft range - you target someone in range, if they fail the save the magic is transferred to the target and range is no longer an issue.
Fireball - the range is only for the point of impact and does not effect the explosion that occurs after the fireball impacts, thus if it impacts at 150ft range the explosion then reachs 170ft mark.
Flaming sphere - range 60ft, gives you a 120 diameter circle in which to control/maneuver the sphere, the sphere can not be sent out of that range, however if the sphere is at the edge of the range the 5ft radius of fire damage can extend to the 65ft mark. Since the magic is still tied to the person who cast the spell, the sphere cannot be made to go further away from the caster than the spells range, however the caster can walk further away from the sphere, in which case they lose control over the sphere. Now It is up to the DM at this point, but a couple things could happen;
- The sphere could just wink out as it is no longer close enough to the spellcaster for it to continue to receive magical energy.
- the sphere just stops where it is, the caster can't control it until they have returned to the proper range, of course the sphere continues to radiate fire damage during this time.
- my favorite, the sphere is now out of control and randomly moves 30 each round until the caster stop concentrating on it, or moves back within range of the spell an succeeds at a DC 12 arcana (intelligence) check (10 plus the spell level).
Just to note I have no real rational for #3 I just like it, and it have proven fun in my games.
I think its best to look at each spell and discuss the ruling with my players, both for their fun, and to ensure balance from a DM's point of view.