To see how much exfoliant you think you'd like, remove about 100 grams of the product you've made and add 5% or 5 grams exfoliant to the product. Mix, then try. If you like it, leave it. If not, add 5% more at a time until you reach a level you like. If you're making a facial product, try it on your face. I have found my hand can take a lot more exfoliation than my face, and that has led to a little too much scrubbiness! Keep really great notes, then do the math on what your skin might like. For instance, if I used 4 grams of jojoba beads in 100 grams of product, I would use about 4% in my recipe. (I realize if you divide 4 by 104 you get 3.85% in the product, but my scale just doesn't have that level of accuracy!)
LEVEL OF SCRUBBINESS
I think we can all agree that pumice probably isn't the first choice for a facial product. Even fine pumice is quite harsh and most skin types couldn't handle it.
Sugar is such a nice exfoliant - it's cheap and comes in so many different grades. But sugar is very water soluble, and adding it to a surfactant blend will create a very sugary surfactant mix as it dissolves quickly.
Salt is another nice exfoliant - it's cheap and comes in many grades, types, and colours. But it too is very water soluble, and you'll end up with a very salty surfactant blend as it dissolves quickly. As well, salt thickens our products, which means you will end up thickening your product pretty dramatically at first, but as it reaches the apex of the salt curve, you'll end up with a really watery product!
Baking soda is another inexpensive and effective exfoliant, but it also dissolves quickly in water.
Some exfoliants like corn meal, loofah, seeds, and shells can get soggy in scrubs that contain water. If you really want to use those ingredients, making a small tester batch and let them soak for a week or so. If they are soggy, consider figuring out the ideal amount and add them right before using the product!
What's left from the list of physical exfoliants? Jojoba beads, clay, dermabrasion crystals, and possibly bamboo and vanilla.
USING JOJOBA BEADS
I've found that using the 60/100 size jojoba beads at 2% to 4% is enough for my skin in surfactant based products. If your supplier doesn't list the size of their beads, ask as there really is such a big difference between the larger beads and the smaller. I really don't like the larger beads and I find they don't wash off easily, whereas these smaller ones are easily rinsed away!
USING DERMABRASION CRYSTALS
I've found that using 4% of these crystals in my cleanser was more than enough!
I think I'd like to write more about this shortly as there are a number of different clays we could use, but I've found that 5% in a product feels quite nice whereas 10% was a bit more than I could handle for my oily, not that sensitive skin.
As a note, odds are pretty good your chosen exfoliant will sink to the bottom or float to the top of the container and you'll have to stir it or shake it to get it to incorporate again. You can work really hard to make it suspend, but I've honestly found that it's not really that vital. We naturally want to stir or shake our containers, so it's not a big deal. If you're selling your products, that might be an issue.
You can see the tiny jojoba beads in this product are lighter than the water soluble ingredients so they have floated to the top. I just make sure I leave some room for shaking the bottle before using! I left this picture large so you could really see the floating bits!