This is probably the shortest strategy I'll ever write: only Double on Spread 7 or better. End of strategy.

Okay, if you're still reading I'm assuming it's because you want a little detail. It's still pretty simple, but here it is: the player only gets an edge when the spread is 7 or more. This is actually quite obvious. At Spread 7, 7 cards will give you a winning hand. And since there are 13 cards from Deuce to Ace (2 ,3 ,4 ,5 ,6 ,7 ,8, 9, 10, J, Q, K, A), that means that only 6 cards will cause you to lose.

Spread 7 gives the player about a 54% chance of winning and it gets better from there on up to around 85% at Spread 11. So the strategy is to only Double on the Spreads that give you an edge, namely 7 through 11. Spreads below 7 give the house an increasingly stiff edge and should be avoided.