Quick Amergraph 150 overview

10 presets and 10 custom sets.
Use the calculator that came with the unit to get started. I forget how it works, but your exposures will change with the mesh count, color, and emulsion. Obviously, if you're used to timed exposures, they're of no help since this unit (and most professional ones) have light integrators which measure light units and adjust as the bulb ages.
You also might want to invest a whopping $7 in a Stouffer scale. That's what I use, and it's been suggested by some pros to use it on every exposure you make.
As for the presets, I only use one, and that's for 305 mesh counts. I've set up my own in the custom settings, and post each on a white board in my darkroom.

I just dug out the paperwork and whatnot that came with mine. Use the "Auto-Optimize" neutral density target that came with your machine. You'll need to do a test for each mesh count you have. You'll also have to do one for each type of emulsion you use, as they all have varying exposure times. I use Ulano QX-1, as it's a dual-cure hybrid that'll last for a year or so in the bucket, doesn't require mixing, has good water-resistance if fully-exposed (post exposure in the sun doesn't hurt, either) works with solvent inks if you decided to get into flat stock work, and has good detail. Reclaims easily and doesn't break the bank.
For what it's worth, I use preset 6 for yellow 305 mesh, coated 2/2 wet coats, with QX-1.

After you've calculated the "best" exposure with the target for the screen you're exposing, take note of the light units on the display, and use that to dial in tighter exposure times and save those in one of the 10 customizable presets (the .1, .2 and so on presets).

One other thing. Buy a spare bulb. You'll have to spring for one from AmerGraph to keep the warranty intact, and they're not cheap, but do it anyway. I had an issue where I thought the bulb had burned out and a replacement was a couple of weeks out (they were backordered), so I used my old homemade unit. The new bulb came in, and it still didn't work. Went through the troubleshooting in the book and noticed that the tiny circuit breaker behind the panel had tripped. Reset it, and the machine would periodically work, or trip the breaker. Finally spoke with a tech while it was still under warranty, and he told me a couple of things, but first was switch out the bulb, even though the old one still worked. I did and has worked flawlessly since. I keep the funky bulb now as backup in case the newer one goes until I could get a replacement, but it goes to show that a bulb can give you trouble even when it seems like something else.

One other thing. Last night I did an exposure test on a new (for me) emulsion that's pretty slow. Used the "Auto-Optimize" test film and ran it on the "0" program, which is the equivalent of 220 light units. I did it on a 195 mesh, which pretty much falls in the middle of my range of mesh counts. This emulsion has a lot of latitude, and I got an acceptable image between the "5" and "9" on the scale, which corresponds to the preset programs on the machine. That said, the actual exposure between 5 and 9 is quite varied, in that the emulsion layer is much thinner on the 5 than it is on the 9, meaning emulsion washed away on the lower numbers, even though there is still enough emulsion present to show the image. When examining your tests, you'll want to use the highest number that holds the detail on that particular mesh count so you retain sufficient emulsion-over-mesh (EOM) in your finished stencil. You also need to examine the 3 different "circles" adjacent to each program number. The right-most circles on each row have the thinnest, tightest lines, and while you might be able to image that circle best on a lower exposure, the stencil will be so thin that it'll likely break down. In a case like that, you need to look at your mesh count and move to a higher mesh count to hold detail and still get enough EOM for a good stencil.
As mentioned earlier, you can note the light units programmed into the pre-programmed sets, and use that as a basis as you proceed burning screens, using the test film with each job and setting up new custom programs with more precise light-unit measurements as necessary.