...and a 3-0-3 step down transformer.

Sorry, I don't know what that is... What's the output-voltage rating or the step-down ratio?

Do you have a multimeter to measure the actual secondary voltage?

Most transformers put-out their rated voltage at full load. A "12V 1A" transformer might actually put-out 12V when loaded to 1 Amp, but 18V with no load (or with a light load).

230VAC is the

RMS voltage. You can look-up the math if you wish, but RMS is sort-of an average*. The

*peak* value of a sine wave is about 1.4 times the RMS value (~plus and minus 320V peak). And of course, the minimum is zero. So with a rectifier, you should be getting "random" readings between zero and 320V (assuming everything is calibrated). Or, if you put a capacitor on the rectifier output to hold the peak, you should read 320V.

...measuring by multiplying by a factor (230/1023)

You should give yourself some "breathing room"... If 230V gives you 1023, then 250V is also going to give you 1023.

* RMS is related to power, and you get the same power with 230VAC as you get with 230VDC. If you hook-up 230VDC to a light bulb, it will glow with the same brightness as with 230VAC.