How do you know what angle or weight you need when fine-tuning your bow with a v-bar or back bar? Does it have to be the same weight as a forward stabilizer?

Much like a lot of other aspects of archery, it's all personal preference. Best advice I could give is to play around with different weights and angles with both your front and side bars to see what feels most stable for you. Bowhunters always like to tinker with different accessories and gear so don't be afraid to try different angles and weights until you get something that works for you.