The resin tank holds about 200mL at the Maximum Fill Line printed on the front of the tank. Be sure not to overfill the tank past that line. It will overflow. A 100mL print will be no problem at all. You should be able to print anything under 175mL without thinking about low resin. If the resin does get low, usually the resin will be thinnest on the left side of the tank, closest to the hinge because the tank is tilting to the right. Be aware that even a print slightly less than 200mL could ‘run out of resin’ due to the resin pooling in the right side of the tank.
A nice feature of PreForm is that it will recognize any print that exceeds 200mL and will ask if you would like the print to automatically pause for a refill before the tank runs out of resin, assuming you start with a full tank. You can then safely refill the tank and resume the print, however, you may notice a thin line in your part where the print was paused.
Adam is right about the resin preferring the downhill side of the tank. I have my printer tilted left ever so slightly to counter-act that so I can print safer. I mostly print small things, so I don’t like to keep my tank full. It’s easier to shake a bottle than scrape a tank.
I do large prints and I find that just over 100 ml MAY go, but, just to avoid overfilling, I slightly under-fill. Given that, after 100 ml there is a good chance that the print will start sucking resin from a silicone layer that has a growing “bare” spot on it. I do not consider it safe to continue to print with only the resin that has been absorbed into the silicone layer. This puts the “real” capacity of the tank much closer to 100 ml, as almost another 100 ml seems to be lost in the silicone.