I'll Echo Lee's remarks and suggest using a densitometer to find Zone I and testing development time on Zone VIII rather than Zone V.

Zone I is determined almost entirely by exposure. Once the Zone I area on the negative has been fully developed, you can increase development time all you want, but you won't increase Zone I density--and therefore you won't increase film speed--by changing the development time or temperature (though you can genuinely increase film speed sometimes by changing developers). The reason the film speed test is based on Zone I, is that you want to find out what the minimum exposure is to effect a significant change in density.

Now the fact that Zone I doesn't change significantly with development time, while higher values do change significantly with development time, is what allows you to use development time to adjust the contrast of the negative to the brightness range of the scene. You test for Zone VIII, because that is the highest value that shows detail, and your goal in using the zone system is to get as much detail as possible on the film at both ends and good tonal separation throughout the range.

When you use +1 development, you want to push Zone VIII up one zone to increase contrast, but note that not every value will increase by one zone throughout the range--Zone I will stay approximately the same, and Zone V will usually increase by some value less than one full zone. That is why one should test for Zone VIII to determine development time, and not Zone V. If everything went up by one zone when you used +1 development, then +1 development would give you no change in contrast.

A couple of qualifiers--

If Zone I isn't developed fully, say when you reduce the development time for -1 or -2 development, then you may want to increase exposure to support the shadows.

After doing all the tests properly, you might find that the film isn't giving you the look you want, so you can adjust your EI to emphasize different parts of the film curve. Say for instance, that you have a film with a long scale (plenty of room before the highlights hit the shoulder) and a long toe--you might reduce the EI (or increase exposure), to push the shadows off the toe.