Mmm, I'm afraid that that's not gonna work (if I properly get what you want).
In order to get 500 ml of water with 0.25 mgPO4/l out of KH2PO4, you'd have to add 0.179 mg (0.25/2 * 1.432) of KH2PO. Then think of how accurate you can measure (both the powder and the water).
For the rest, doubling the (milli)grams will do
Mmm, isn't
this dutch calculator yours (I might be mistaken)? You can use it to check my calculation...
If you'd insist proceeding as above, you could make a 2mg/l solution (so 0.179*2*2*2 = 1.432 mg KH2PO4 in 500 ml water). Then split the stock in half, and add 250 ml of water to one of the parts. This one will be the 1 mg/l solution (the other one will stay the 2mg/l solution). In same way, split and dilute the 1mg to 0.5 mg/l and subsequently to 0.25 mg/l
In any way, try to assess your accuracy/error, also in case you decide to make a bigger batch...
Groetjes,
Riba
