Making them configurable would be pretty straight forward, but I've spent some time making sure to get the default numbers the best I can, so I'd like to talk to you about why you think it "should be much more".
First, you should realize that my final percentages are based on what is needed for 1k spm. If you add up the different machines and then adjust for the crafting speed, you'll get almost exactly my final goal ratios. When I first crunched the numbers I got that refineries should be about TWICE as rare as they are now, so I ended up halving their power and doubling their spawn rate, so they wouldn't be TOO rare.
But yes, the intention is for them to be rare. Something you have to hunt for. And I purposely left pictures out of the mod so that it would be a fun thing for players to discover and marvel at how big it is. I actually originally had a +2 offset to delay find them, but realized that wasn't good because by the time people had trains and were actually using my factories, oil was actually something that is right around the corner and needed.
And potentially digging through my code you've already figured out the reason for the assembly offset is because people don't need the factories in exact 1k spm right away and actually more just need different types of assemblers, especially before they are saturating most of them, so making sure they have about an extra 30 early on seemed pretty reasonable to give them a variety of places for crafting the different recipes before trying to give the player perfect 1k spm ratios.
So, suppose you or I were to increase the refinery ratio. Then you'd simply end up with many more refineries than you need. Another way to do it would be to further decrease their power and increase their frequency, but they are such a pain to deal with I'd much prefer fewer more powerful ones. Finally, I could give them a negative offset so they're found earlier, but I just think that takes away from the experience of discovery. I've started many games with the current setting with the express purpose of seeing how long it takes to find the first refinery, and sometimes it takes a while, but it never seemed too bad to me, though I can respect that others would have a much different opinion than me.
