With the baby aspirin, I follow the dosage suggested in The Joy of Chinchillas: mix 1/4 of a baby aspirin(ground) with a small amount of a substance that your chinchilla likes. (My chins love rasberry syrup, so I use .5 cc of that). This is given 6 hours after dental work. They say that it's safe to give this dose every 12 fhours for 48 hours, and I have found that to be true with my chins.
I've never seen a suggested dose for the infant simethicone, so I give the smallest measurable dose, which is .1 cc per chin. My vet tells me that this is a benign medicine that breaks up gas bubbles in the gut. You can hear a grumbly tummy by listening very closely with your ear up against a chin's stomach. You can hear some interesting squeaks and croaks - it's kind of hard to describe. I had one chinchilla, whose stomach growling I could hear from across the room, it was so loud. It was so chronic that I started feeding him a liquid supplement of ground hay, oats and nutritional shake mix each night, and he finally got over it. Generally though, I've used simethicone(in conjunction with reglan and probiotics) whenever I thought we might have a case of bloat at hand with one of our chins. That, thankfully, has been very rare.