You might want to consider making a tincture of those spices with Vodka. Let it soak for a week or so. Then run it through a coffee filter to remove the spent spice particles. Then add measured amounts to a glass of the beer until you get the amount of spice you're looking for. Then calculate what the whole batch would need, and add that amount. Everyone's idea of enough spice is different. The only drawback is that some brewers don't like the heat of the Vodka in the beer. In a pumpkin spice beer, I'd say it'll be less noticeable.