Half a blotter generally doesn't decrease the length of the experience, just decreases the intensity. So, you'll still technically be tripping at the 12 hour mark but its much more dulled down. Half a blotter is honestly my sweet spot. I sometimes get some cool visuals, a heap of different ways to think, extreme euphoria/body tingles etc and it always seems to go for the perfect amount of time. By the time i'm thinking "alright, i want this to end now" it normally does.
As for light shows on LSD, I've never had one, but lights in general are amazing on it. A light show could be great but if it is your first time it could be sensory overload and could wig you out. No-one really knows how you'll react, but in my personal experience, half a tab of blotter is almost always a safe way to trip, can be fun and light-hearted, and yet insightful at the same time. You might get a few negative thoughts, but you're still there in your mind to dismiss them as silly. Goodluck!