The heat of peppers varies from batch to batch even in commercial ground products, so it can be hard to say without tasting it. The way I typically approach a spiced beer is to make a tincture with vodka and spice. Let it sit for 3-7 days or so, then strain off the vodka. I then pour 2 oz samples and start dosing them with tiny amounts of the spiced vodka, but I can accurately measure very small quantities (e.g. 1 microliter at a time).
Since most people don’t have the tools for that, I would suggest pouring a 2 oz sample (Sample A) and adding some easily measured quantity, say 1/4 tsp. Make sure it is evenly mixed in. Then pour a bunch more 1 oz samples (Samples B through F for example). Take 1 oz from Sample A and mix it into Sample B. Mix well. Take 1 oz of Sample B and mix it into Sample C, etc. You are making serial dilutions and each will have 1/2 as much of the spiced vodka as the previous one, so if you go out through F it will have 1/128 tsp in it, which is ~39 microliters in 2 oz. When you taste them, taste them in reverse order so you don’t blow out your taste buds right away. You might need to dilute it further, and if you do I recommend waiting at least an hour. Even if you think you have it right it is helpful to come back another time due to palate fatigue. In my case, I have other people do the tasting for peppers because of my heat tolerance (which is really not crazy high but higher than a lot of people). You can always change the dilution rate to 1/3 or 1/4 or whatever you want, you can get to very small quantities quickly that way, but you won’t have the granularity you may want to zero in on a quantity. In that case, you may find you like something between a 1/100 and 1/200 dilution, dilute to 1/100 and then create 1/120, 1/140, 1/160, 1/180 dilutions to try to nail it.
Anyway, once you figure out how much you added to the sample you liked, figure out how much you would add to an entire keg. And remember, it is that amount in 2 oz even if you took out an ounce. If you have enough vodka, I would just add that at packaging, -20% (because it is easy to add but hard to take away). If you don’t have enough vodka, figure out how much cayenne you would add to the needed vodka to have the same heat. Then skip the vodka and just add that amount of cayenne to secondary (again, -20%). If you are adding spice directly to the beer I would taste it daily and rack it as soon as it tastes the way you want it to.
I have gone through more than 30 iterations for some beers to try to get the spice level right. It can be a pain, but it saves on time and wasted beer in the long run. Feel free to use a commercial beer for the experiment, as long as it is similar to what you are brewing you should be happy with the results.
Or, you know, throw in a tsp and see if you like it ;) As long as you taste it daily it will be fine.
