How much unused supply should I keep available as zerg for a macro-focused build?

Almost impossible to tell you. A rule of thumb is probably number of larvae < unused supply. But even looking at that doesn't seem like a good rule of thumb to me.

If you are in a macro style game, making a 4-5 overlords after your second or third batch of drones shouldn't be that much of a resource issue.

I would probably tell you to scout earlier than to make extra overlords. If you scout and see an army coming, it doesn't matter what race you are, you're probably going to be in trouble anyway unless you blindly prepared for it.

You are right to pump drones for as long as possible before building an army, however your opponent's goal is to either prevent you from doing this or to catch you off guard with an attack that you are unprepared for. So, keep practicing your scouting. If it is successful, you'll have a sense of how big his army is and what it is comprised of, and will therefore always know the number of overlords, units and larvae you'll need in order to feel comfortable in the event that your opponent decides to move out.

Keeping in mind that you can often force out a round of units by the time the army reaches you, the unused supply you have available should be equal to the amount of supply you'll need to defeat his army, minus your existing army supply. This calculation depends not only on size, but composition as well. Given that you know what his army is comprised of, due to your scouting efforts, you'll know which units to build to most efficiently tear through their forces, and should be able to estimate how many of them you will need.

One thing that might work in a pinch: If you find yourself supply blocked with a large army marching toward you, send out whatever meager army you do have to meet it and harass it while you spawn a few more overlords and some units to replace the ones you lose. A few ravagers can certainly slow down a march with their corrosive bile.

Alternatively, hit a soft spot if you can find one, such as a newly-saturated expansion. You'd be surprised at how often players (even pros) will draw their entire army back to deal with a handful of zerglings and roaches.

just depends on your APM(Actions Per Minute) and whether or not your going for simple units like ling bane are going for ling/roach overlords are useful for scouting and can easily be changed into overseers. supply blocking usually involves the other player's attack on your overlords. in short keep your overlord making till about 3 or 4 supply is left and your injects mean nothing to your supply since you would not want to use up all your larvae to make and army all at once, at that point you've lost the game.