Well if you want to improve your macro then it varies, what race do you play?
If you're zerg chances are you need to realise when there isn't much micro to do and just a-move and ignore the battle while macroing.
If you're terran: chances are not that you forgot to build units, its that you forget to add production at key points. E.g. TvP: 1 base= 3 rax, 2 base = 5-6rax, 3 base = 7-10 rax etc etc.
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