Hi Marco,
You are correct, ohms law (E=IxR, where E = voltage, I = current and R = resistance) determines what happens with current voltage and resistance. If you have a fixed voltage, to obtain a variable current flowing through the tank you need to vary the resistance. The way ohms law works is to imagine a triangle with E on the top and I R on the bottom. Therefore if you want to know the resistance required at a known fixed voltage and a known current the formula would be E/I = R (Voltage divided by Current = Resistance.
So ohms law is:
E = I multiplied by R.
I = E divided by R.
R = E divided by I.
Where it gets tricky is power, you need to make sure the resistance wires you have can cope with the power. In the above example the resistor setup must be able to disipate 12 watts.
Take a look here, there are online calculators for working out everything ohms law.
Ohm's Law Calculations With Power
Now back to the voltage versus current calculations for electroplating. With no additional resistance in a plating setup (just a good power supply and wires) and good plating solutions with the correct amount of brighteners and wetters all plating can be achieved by aplying a voltage. I never calculate how many square inches any of my items are. I always set voltage to what I know is the correct range for my tanks. For nickel and nickel cobalt it is 3Vdc, for zinc it is 0.8Vdc. The only time this differs is if I am plating a very small piece, I then use 2Vdc for nickel and 0.5Vdc. I have plated a single small screw with less than 1 square inch of surface area at over 0.5A and it came out fine because my solution was correct.
The majority of commercial nickel plating solutions have an operating window of between 3Vdc and 6Vdc, most commercial chloride zinc setups operate between 0.5Vdc and 1.5Vdc. I can take a bracket of say 50 square inches surface area and with absolute confidence apply 3Vdc to my NiCo (copy chrome) setup and leave it for 15 minutes and I will have a nice shiny defect free nickel cobalt plated part because I know that my power supply will provide the current that item requires automatically due to ohms law, provided i have placed the part equally spaced between the anodes in my tank.
I know this is all a lot to take in and I am rambling a bit but I have seen so many posts on here where people (including myself in the early days) had failures with copy chrome and zinc. Items rusting or tarnishing within days of being plated, dull patch plate, black streaks etc. This was because our solutions were wrong and we made them wrong. We didn't take the time to first plate small test pieces. The smartest thing I ever did was to take the copy chrome solution I had got completely wrong over about a month of mucking around by adding acid, brightener, more acid, more brightener and putting the whole lot through a charcoal filter and baselining it. I then added what was the correct amount of brightener and hey presto everything started to work fine. I now track how many amps I plate per hour and add brightener at the correct rate. The only time my tanks ever get it wrong is when I have a failure in plating due to bad prep by me of aluminium or zinc diecats parts and my tank gets contaminated.
My advice is to get a benchtop supply that displays current and has variable voltage, or get a variable current supply. This will allow you full flexibility to do everything you need to do. It is money well spent.
Cheers
Mark