Your DHT levels do not matter beforehand, the only reason people get it tested before is if they crash in which case they can get further tests and compare the two to see what went wrong, (e.g. if they're previously inhibited DHT has begun to rise again or if it has stayed at the same levels even after quitting there AA).
You will have the same risks of sides as everyone else, maybe slightly less if you're taking .5 a day although I don't really see the point of not just taking the 1mg, IIRC the efficiency of .5mg was shown to be nearly that of 1mg so the side effect ratio would be near the same as well. You're biggest risk of sides is psyching yourself into getting them by over thinking this. Like that 16 year old who took Saw Palmetto twice and "ruined his life"...