EDIT: Please read the space booze thread, as some of the assumptions in this post are dated. For instance, nutmeg from India has been shown to have either a lot or very little allylbenzene content - country of origin is not the best indication of potency. Additionally, mace also varies widely in allylbenzene content. I would recommend using color as a better indicator of potency (the nutmeg should be a red/gold color).
downwardsfromzero : Nice link. I've seen the site before but hadn't really looked in depth. With space paste, I started with 2 tablespoons because that's what the original instructions suggested. Again, just from my preliminary research, the main enzymes being inhibited are MAO and CYP3A4. Now mace is usually stronger than nutmeg because it has a different compostion. Nutmeg has a large portion of the total mass in non-volatile substances. Mace has a higher volitile oil content on average. So if you want to be safe, you could take 1-1.5 tablespoons to play it safe. Nutmeg has a lot of stuff in it that contributes to body load. Mace has less of it, but still has some. It has a much stronger taste, and an aroma that is similar to nutmeg, but again stronger - sometimes I get nausea just from smelling mace; mixed into space paste, it will probably taste better.
I can't stress enough how important it is to add the nuts. I've tried a few times making the stuff without nuts and I always get sick, and have a much less *interesting* experience. Seems from my research that pistachios have flavinoids that *may* (theoretical) give you a better trip; i'm not completely sure why, some on another forum that is much less evidence based have theorized that estradiol 17 dehydrogenaze has to be induced to have a really good trip, and the flavinoids in pistachios do in fact induce this enzyme - again, this is hot air at this point, but without pistachios it doesn't work right. Almonds will passively induce aldehyde dehydrogenaze, which also seems to be a good thing. This might be why you are supposed to soak the almonds, because it will create more aldehydes in the nuts, and why we have cumin and cinnamon in the mix.
IF THERE IS ANY WAY TO TELL: See if you can get nutmeg that was native to India. Indian nutmeg has very high allylbenzene content, and the allylbenzenes in nutmeg, at least according to the studies performed on nutmeg toxicity, are the most likely hallucinogenic fraction.
Pretty much all the spices also contribute terpenes which are shared with the cannabis plant, and modulate the experience and also contribute certain mood components. This may be why space paste works better and has less negative effects than straight nutmeg.
A side note, when I was younger and took nutmeg regularly as a cheap legal high, there was one store in my area that had ground nutmeg that always made me trip to the point that I was completely out of my mind. That store no longer carries nutmeg, but I'm going to find out what their supplier was if possible and I'll post back if I can find it.