The question of "How much should I use" is often brought up here on TF,
especially when using some fertilizer or substance that is of questionable toxicity to the plants.
The answer is always a variable, based on how large the plant is (not necessarily only on the amount of soil), what type of plant it is, and more importantly how well the plant is growing & what your other conditions are.
A sick plant cannot utilize a lot of any fertilizer type, and a plant growing in "less than ideal" conditions cannot either. So when looking at "how much" of anything to use, most often that decision is best determined by the person growing the plant.
(I am not directing most of this post to you Gill_za, but to growers in general.)
Problem is, a lot of people (new growers in particular) want to find 'secret' ways & new tips on how to force their plants to grow faster & bigger than normal, and as I often bring up, PATIENCE is often the best ingredient you can give your plants!
(A Cephalotus is a good example. People get one or transplant one they just got, and then while the plant recovers from transplant shock or is adjusting to its new surroundings/conditions, the grower gets worried & impatient because the plant is looking poorly or not growing, so they begin watering the thing until it rots, or fusses with it non-stop until they kill it!)
Back to "how much"... in order to figure that out, I often do a "search" here on TF to look to see if anyone else has already dealt with the situation. (Indeed, when dealing with Pings for example, there are a LOT of things you can grind up & put on your plants!)
Next I look into fertilizers in general to see what amounts are used in normal fert's for common house plants. (I then always start with a lot less on my CP's, to see how they react!)
Ahh, did you catch that? I said "to see how they react!" That is what YOU are responsible for! That is where the real growing experience comes from. That is how one becomes a good grower! To learn to listen & watch your plants & their reactions to what conditions you subject them to! One does not become an expert at growing overnight! That's like dousing your VFT's with fertilizer and expecting to force them to grow fast & big... it's foolish & it isn't going to happen!
So while in the case of Iron Oxide (or powdered rust), there are recommendations I could give,
(Gill_za, I will talk to you more about it in a PM) I would simplify it & just say to use a small amount, like less than 1/8th Teaspoon per pot. Mix some in water & dump it quickly into the soil media so the small undissolved particles get in.
Indeed, chloriosis is a problem for plants at times, and iron sulfate or chelated iron is the most usual solution to the problem. Keep in mind that depending on what plants you are dealing with, may also determine how much of these you apply. Iron oxide isn't very problematic (or chemically "active") from what I have seen, but then it may also not give the best results for correcting chloriosis.
I recommend a small amount like this (or smaller depending on what is being used) because adding anything to a plant that isn't getting the right amount of light/moisture/proper temperature/etc. is not going to help the plant anyway & in some cases may be enough to kill it. And also, if by chance you do see an improvement, then you can always add a bit more after a few weeks, AFTER you see how the plant responded to the first application.
You only need to feed a plant an amount that it can absorb at the time you apply it (for the most part). Adding enough to truly enrich the soil & hold the plant over for a few months is something reserved to common garden plants.
I regularly read here on TF about some new grower who applied too much fertilizer, insecticide or other such chemical/growth stimulant/foreign substance that is supposed to somehow help their plant, & they then have the situation of having to flush the substance out or repot their entire plant to somehow save it after exposing it to too much toxin.
Better to start with little than too much!
Again, the only thing you can safely go a little overboard on with applying to your plants, is patience!
Lastly I want to remind people that the very first thing they should do is to look carefully at the conditions you are giving your plants, and improve on those if they are lacking. (I am not suggesting to go out & burn your plant under 1000 watts of lighting, as that is NOT an improvement!) But so many people here do not have conditions that are even near "ideal", and when their plants don't grow healthy & robust, they immediately start thinking about "feeding" it with some sort of fertilizing concoction!
Any fertilizer/food/etc will NOT make up for lack of lighting, poor watering habits, incorrect temps or humidity, etc. So make THAT your FIRST priority! Start giving the plant what it really needs, in order to grow its best, its fastest, & most importantly, to keep it the most healthy that it can be.
There is a vast difference between a plant that is thriving, & one that is surviving.
Good Luck all!
PS: The reason I got into these other things that many may feel are a bit "off subject", is because I find it hard to believe that there are that many people who have real chloriosis problems due to lack of iron. I tend to believe that most people have their plants in sub-standard, less than ideal conditions (a much more common event) & that's what the plant is actually showing symptoms of. 
