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Assuming you want equal physical and special bulk, you can do a little math analysis on it.
Assume your mon has T hitpoints and 0 def and spdef point. In order to increase each of those defs by x, you need to substract 2x from T, leaving you with T-2x HP. The damage formula contains a lot of factors, but boils down to A/x + B. Therefore your mon will lose (A/x+B)/(T-2x) relatively to its max HP from the attack. Its derivative has a single positive zero of [sqrt(4A^2 + 2ABT) - 2A]/(2B), which denotes the optimal x you should take to minimize percentage you'll take from both physical and special attacks.
Now, both A and B contain extra modifiers not directly related to attacker's level, attack stat and move's BP. Getting rid of it, you're left with new B being equal to just 2, and new A (let's just call it C now) equal (2*level + 10)/250 * Attack * BP. Therefore optimal x is now [sqrt(C^2 + CT) - C]/2. If you now withdraw C from the fraction and approximate resulting sqrt(N) to 1 + (N-1)/2 (taylor series around N=1) where N = 1 + T/C, then you simply get a result that the most optimal x is approximately T/4.
If you don't care for either physical or special bulk and just want to max out the other one, then correspondingly you get optimal x of T/2.
Summing it up, you should aim to have twice as many hit points as your def and spdef points if you want both bulks to be decent, or your hit points equal to either def or spdef points if you want best possible physical/special bulk and don't care much for the other one.
Going for extremes such as with Shuckle is obviously bad. Try relocating some of its Def and Spdef base stats according to the above conclusion to see how much it improves.
