The standing (quiescent) d.c. level on the mid-point should be half the d.c. supply voltage that you’re using. That’s half of 3.3 V for an emonTx, and half of 5 V for an Arduino (or most Arduinos).
One question that I forgot to ask about your transformer: what does it weigh? If it’s a copper and iron isolating transformer, and it runs a 20 or 25 W lamp, it has to be 20 - 25 VA minimum. One of those will weigh at least ½ kg (I’m looking at the RS catalogue and a 20 VA weighs 0.55 kg and that’s without a casing around it). I have a 12 VA one that weighs just under 400 g, and a 50 VA is 1 kg - so weight is not directly proportional to the rating but it’s a good guide. If yours weighs substantially less than 500 g, it’s got to be an “electronic” lighting transformer that is in essence a switched mode power supply with an a.c. output, and one of those is totally unsuitable for what you’re trying to do.