Dragons can use their breath weapons once every 1d4 rounds, so breathe wait breathe. A cone projected on a plane is a circle, and at it's terminus you'll have a 40 foot circle. If the approach is at an angle you'll get an ellipse. And it is typically the hubris of a dragon that gets it into trouble. Why worry about those puny humans? But strafing runs are a thing (flyby attack), and attacks from a party that can't range just makes good tactical sense, even if it sucks for the party
But in the specific monster description, no such limitation appears:
You also need to refer to the breath weapon entry in the general rules for true dragons: http://www.d20pfsrd.com/bestiary/monster-listings/dragons/dragon/ 'A dragon can use its breath weapon once every 1d4 rounds, even if it possesses more than one breath weapon.'
Breathe, then roll 1d4. Breathe again, roll again. And so on.
A dragon worth its salt will use and abuse such tactics as long as they're effective. If your dragon purposely closes to melee range, something has gone horribly wrong.
That said, white dragons aren't quite as smart or wise as some other types, so there's room for a bit of arrogance if it thinks it's a certain victory. I'd never play an ancient or older dragon that way, though.
Dragons are clever adversaries, even the less clever of them like white dragons is cleverer than the cleverest cleverdick amongst the players.
The advice to start the dragon in the air and keep it there as long as possible is excellent. Have it fly by, breath, and then keep flying, taking a few rounds to circle back and breathe again. Once the PCs start to take cover, have the dragon try to pick off the most isloated and vulnerable among them, clawing from the air and trying to grapple a character so that it can devour him before going back for more.
All too often, dragons are treated as punching bags by players. Just don't let them close and get all their full attacks at once. Have the dragon use its mobility to the fullest, and even retreat if it takes some damage, then plot how it can ambush them even better the next time.
Generally speaking you are correct that dragons don't have grab. Which means the dragon will make a grapple attempt, not a claw attack.
Anyways, the dragon should try to avoid getting into melee range of the party for as long as possible.
It should probably make extensive use of it's fog cloud ability to block line of sight from most of the party while using it's breath weapon to target individuals who exit the fog. Unless the PCs can find a way to force it to land and get into melee range, it simply shouldn't.
When I mentioned a flyby attack, I meant the dragon. Flyby attacks (a feat that the dragon has) allows the dragon to take a standard action anywhere along it's movement path, in addition to it's normal standard action. So a particularly mean dragon might cast fog cloud, move and breath and continue to move out of range.
When I mentioned a flyby attack, I meant the dragon. Flyby attacks (a feat that the dragon has) allows the dragon to take a standard action anywhere along it's movement path, in addition to it's normal standard action. So a particularly mean dragon might cast fog cloud, move and breath and continue to move out of range.
Yeah, this is what most dragons should do. Never let the party get close to them. Focus on taking out spell casters over non-casters (if obvious). Of course, the breath weapon will usually be able to get the entire party so that's rarely a decision that needs to be made.