Firstly regards the ammonium....
I can't source ammonium thiosulphate here in Australia so I had to go with sodium tthiosulphate instead. I have no direct experience with the difference but all my research tells me the the ammonium thiosulphate is far more effective as a fixing agent as it acts much faster that sodium thiosulphate.
So in short, if you can get ammonium thiosulphate then definitely go with that.
As to the mg measurement well my first batch I (luckily) guestimated it!
Yesterday I asked stefan4u the exact same question. His answer is basically the same as what polyglot suggests, ie make a dilution and use that. I will certainly be using that method next time I mix.
Stefan4u's answer below: (which is more or less a variation of what polyglot suggests)
"Take one liter of aqua demin and add 1 gramm of Potassium Iodide. 1 ml of this soulution will be 0.001g or 1 mg (1.4mg will be then1.4 ml).
Including all deviations in measurement this is the best / only way to do this at home...
Than take a small syringe and you will be able to come very close to the afforded ammount."