ok so here's what we have. the iron test tube on the left, and the hardness test tube on the right with sample level 1/2" below the top. the top of the green tape is the 1/2" mark. one packet of reagent in each, shaken. same water in both.
Note how the smaller tube appears slightly lighter in color. this makes sense if you think about it... less water depth in that direction. The sample measured 0.6ppm with my color disc, and when I dumped the sample from the smaller hardness tube into the iron tube it exactly filled up to the line and also measured 0.6ppm. However, when you compare it to the online pics of the color disc (both via the photo and by holding up the tube to the screen) it does not align with 0.6ppm... more like 0.4 or even slightly less. I chalk that up to online rendering of photos, slight changes in brightness/contrast etc. and just the difference between using the viewing box and not.
So... yes this will work for you if you are not looking for highly precise measurements. but your readings will be slightly understated. if this is too much hassle for not enough accuracy then go for the full kit.