I would second what Dave says. 3mm acrylic is pretty much our standard material, and (allowing for the different tube in our 6090), his suggested settings tally with what we use.
The ramp test is very simple - just cut a simple shape over and over from, say, 10mm below where you'd normally focus, raising the focus a little each time until you have found the "sweet spot" where the width of the cut is narrowest. Then measure between the workpiece to a convenient point on the cutting head. I usually then cut out a square of acrylic that size and stick it on the keyring with our machine's keys (otherwise, I guarantee it'll get thrown away as "scrap" before long!). You can then repeat the same thing with different power settings and speeds to find the sweet spot for those too.
It's a bit time consuming, but you shouldn't have to do it too often - once for each new material you want to cut is usually enough.
Couple of other things worth noting...
- It seems a bit perverse, but turning UP the power isn't always the right thing to do if your parts won't pop-out. Firstly, it gets the plastic hotter around the cut, so it's more likely to sink back into the cut and fuse back together. Secondly, too much power can sometimes cause the plastic in the cut to change its properties - forming a kind of hardened barrier at the bottom of the cut that prevents the laser going right through. A Brucie Bonus is that you save on electric bills and your laser tube will last longer!
- Get yourself a thermometer and check the temperature of your cooling tank (Assuming you have one, I'm not familiar with the 3020) - it's a pretty good guide to how hot the tube is running. Once the temperature gets above about 20-25degrees, the laser loses it's efficiency, and you'll get much less power than you asked for. When we added a chiller to our 6090, the difference was amazing - from trashed workpieces after only half an hour, to utterly reliable cuts running continuously for a full 8hr shift. If money is a bit tight, get yourself some wine cooler blocks, stick them in the freezer overnight, and add those to your tank at the start of a job!
All the best