The easiest way to spend money is to inject and make overlords.
Let me say that one more time. INJECT BUILD OVERLORDS
If you remember those two big statements while you play, you will win a lot more games.
Also, keep in mind that in ZvT you'll definitely want a macro hatch, assuming a ling based style, whereas in the other matchups your larva requirement depends on your unit composition (roaches are larva efficient, lings aren't).
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