Speedlite zoom setting in flash modifiers and camera ISO (with test results!)

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A couple of questions:
1) are you using ETTL?
2) how are you calculating the correct aperture to use? Are you using a light meter?
I find that ETTL never works properly when I use speedlites with light modifiers and it often results in a shot that is under exposed. The camera's meter is easily fooled, for example if your model moves slightly or if you change the composition.
Using a light meter will allow you to set the exposure accurately and then, once set, you don't need to change it. Just leave the speedlites on manual and they will produce the same amount of light for every shot. You only need to meter again if you move the speedlites or if you increase or decrease the power.
It will also let you see the ratio of flash to ambient light and it will allow you to alter the balance between your speedlites, for example if you want the key light to be more powerful than the fill light.
The zoom setting really only changes the character of the light - so use a telephoto setting if you want to create a highlight on your subject, or a wide angle setting of you want a more even spread of light.
When using a softbox or umbrella I leave my speedlites set to 35mm and I find that works quite well.
With regard to the placement of your speedlites - you do need to put them quite close to your model, but this is because it will give the light a much softer, shadowless character. Placing the speedlites a long way from your model will create harsh shadows, which is usually not the effect you want to achieve. I put my speedlites as close as possible to the model, so they are only just out of shot.
Of course a speedlite is usually less powerful than a studio strobe, but that should not be an issue unless you are placing them a long way from your subject

Thanks Ian,

1) I shoot in manual flash control and manual mode on the camera so that I have full control of how the exposure.

2) As I don't own a light/flash meter, I just judge the exposure from a few test shots and their histograms. I would like to have a light meter and shopped around a bit for one, but I can't really afford one at the time being so I'll stick to the trial and error method for now.

I understand the relationship between source size/distance and the quality of the light, but I was initially shocked that on full power using a 33'' reflective brolly box, I had to place the flash at less than one meter to obtain a decent exposure. I felt that there was not much room for composing the shot. However, It appears that a fresh set of batteries and being rigorous on testing solved the problem. It appears that for a head shot, I have a decent exposure at 1/4 power and the light at 2m from the subject.

As for the effect of the zoom setting on the light harshness, that is precisely what was the concern of my initial post. Now that I've looked at how light is diffused inside the brolly box, I think it would be worth exploring how this affect the light falling on a subject. Nevertheless, in the immediate, I'll set the flash zoom to 24-35mm in order to make the light as soft as possible.