When you define keys, assign the right mousebutton to the flashlight. It works wonders for me, so I'm never afraid to walk around with the flaslight in the dark anynmore.

If you think about it, how often do you really need to zoom in? And when you have to zoom, you usually have plenty of time anyway, but you don't have plenty of time when you have to switch from flashlight to gun.

The zoom is generally too weak to be of any good use, and besides it's still there at the Z-button.

For me, the default right mouse button is jump, and the middle mouse button is zoom.

It doesn't really matter what key you put it on, that is relative to your setup.

What does matter, is that you make sure you have Flashlight-Last Inventory key set to something easily accessible -- you ever played Counter-Strike and were an AWPer then you know how to setup the Last Inventory key for quick changes.

I use Mouse 4 and Left ALT personally.

The purpose is to switch to the flashlight when you need it, quickly sometimes and then to be able to switch back quickly upon needing it. If you encounter anything other than a zombie or flaming head thingie you will need more power than what the flashlight can dish out.

When walking around, you can hold your flashlight, but pay attention to the noises and as much light as you can, before entirely going into a new area you should scan the room. For example, if you open a door and there's nothing in front of you take out your flashlight, scan the room slightly peeping in and look left, right, high and low. Most rooms will be empty in the main areas, but memorize what is in the room. Was there a shelf on the left? What boxes were on it, are they blocking the way if you have to shoot? What was on the right? Is there an extra set of bricks jutting out from the wall? There could be something behind it, or in a corridor there. Are there stairs? What is under the stairs?

Paying attention to detail is the key to mastering the almighty flashlight.

Or you could just spam around a grenade and light it up with the chaingun, lol~