Is the half hour that you are supposed to make sure the candles are lit Shaa Zemanios or exactly thirty minutes?
Kitzur Shulchan Aruch (139:10) gives the rule about half an hour without any further specification, and then further (in the same paragraph) states that the earliest one may light is at plag haminchah, which he defines in terms of shaos zemanios. This would seem to indicate that, at least according to him, this half hour means just that - exactly 30 minutes.