I've seen that alot of people will go to great lengths in trying to get a surface volcano that also has flux. Under normal circumstances, this is a very rare combination. With vanilla DF, not only are volcano sites very rare to begin with, they also tend to have high volcanism in the surrounding areas which eliminates the chance for sedimentary rock layers where most fluxes are found.
I've figured out how to use PW to get sites with both. What you want is to have low volcanism tiles right next to max volcanism tiles. The best chance is to have single max volcanism 'dots' as potential volcanoes that are surrounded by non-igneous extrusive tiles. To get that most easily you can max out the weight, frequency and smoothness controls on the noise input, and also set the igneous extrusive threshold to be the
same as the volcano threshold, such that all igneous-extrusive tilse are also potential volcanoes. Here is an example of what you want to see on the volcanism tab in PW:
See how there are many isolated red dots? If a volcano appears on one of those dots, you will have a good chance of finding a site square that has a small island of volcanic layers surrounded by biome with a sediment layer. Keep in mind that with PW you can usually boost the number of volcanos without fear of map rejections. As long as there are more that 200 red tiles on the volcanism tab, you can turn up the number of volcanoes in the world-gen params to the max of 200 without trouble. The more volcanos you have, the more likely you will find the site you want. You just have decide if it looks good on your map. Not everyone wants a ridiculous number of volcanoes.
(Note: The above example is for generating a large map, if you are genning a smaller map, you'll want the red dots to be bigger so that they don't dissappear when the map is shrunk. In that case you will get rejections! If you are getting volcano rejections that is why. Just increase the size of the noise features by lowering the size slider.)
On the first map I genned, it didn't take me long to find what I was looking for. You still have to poke around, as many volcanoes will still appear in the middle of other volcanic clusters, and also because low volcanism does not guarantee a sediment layer.