You can predict to a certain degree, but it depends on rigidity a lot, and the tool insert grade, geometry, corner radius etc. Usually, you start programming a CNC machine after you have a few years of hands on experience using the machine, so you already have a pretty good idea about your tools and machine behavior in various materials. But like I said, the tool manufacturer will give you good start points, which you will play with to get exactly what you need. It's not too hard, just start with manuf. specs, make a part but leave material on the face so you can take multiple cuts without needed to redo a new part for each test. You can still get the 1st part to your liking if you have enough material to face off. Usually a facemill with give a very flat face, if you need it rough, you can use it with only 1 insert so that it leaves a rougher surface, but generally, if you go with manuf. specs, you'll get a very nice below 20 finish most likely. But then again, you said you don't have the surface finish measuring tool, so how would you actually know what you got? If you need to respect a certain surface finish as per part drawing, you will absolutely need that measuring tool, otherwise you won't know exactly what you got. You'll be close, but not exact.
