My HRV score drops ( around 25-30 ) pretty significantly when standing compared to supine. I take both readings in the morning one right after the other. Is it normal for my HRV score to drop so significantly just by changing positions?

Take a look at this: http://hrvtraining.com/2012/08/27/supine-vs-standing-hrv-measurement-is-one-better-than-the-other/

Great article. I started in the supine position and thus for the sake of consistency I will stick with it. It will be harder to notice patterns if you keep changing your position with each reading.