If the Iodophor you have brought is properly labelled it will have the concentration of available Iodine, from there you should be able to work out exactly the right amount to use.
There is a very handy little equation that anyone using chemicals should know called the standard dilution equation it goes like this.
C1 x V1=C2 x V2. Where C is Concentration and V is Volume
We are lucky to be working in metric it makes all the units easy, but it is very important to use the same units all the way through the calculations, in this case we are working in Milligrams (mg) and Litres (L). As a mg is a thousandth of a gram and there are a thousand grams in a litre of water it turns out that 1 mg/L is 1 PPM.
To use a practical example, the Iodophor I sell says on the label, Iodine 20g/L, the first step is to get that into mg/L, as there are 1000 mg to a gram, the concentration of the undiluted Iodophor is 20g/L x 1000 or 20,000mg/L
This becomes our C1 (20,000 mg/L)
Lets say we want to make 5 Litres of Iodophor at 12.5 mg/L, thats our C2 and V2.
What the equation gives us now is:-
20,000mg/L (C1) x (volume 1(L)) = 12.5mg/L (C2) x 5L (V2)
At this point it might be easer to change that 5L into 5,000mL, because we will be measuring the Iodophor in mL, not Litres.
Simply rearrangement of the equation lets us work out exactly how much raw Iodophor we need;
V1 = (12.5mg/L x 5,000mL) / 20,000 mg/L = 3.125 mL
Read your labels and get the dilutions right and you will get good sanitation, no bad tastes and you will save money by not wasting your chemicals.
Naturally if the Iodophor you are using has a lower concentration of Iodine you will need to use more, but just plugging the numbers into the equation will get you the right answer.
Hope this helps
MHB