Alright, to your questions:
No, my system is not based on a math formula. I tried this at first, but it failed miserably because the gold values given in the book have no mathematical correlation with how much time they should take. If you look at Nifft's system, it works pretty well for high gp equipment, but for anything under 1000 gp it doesn't model too well. I basically lumped all the equipment into categories of time that they should take.
As for the alchemist fire, you calculated it correctly, its 5 days. Under teh normal system, assuming a +10 craft skill....you could make an alchemist fire in 3.5 days. In general, you'll find in my system the time for all crafts tend to move towards an average....you can't make a whole drawer of silverware in a day, and it doesn't take you years to make one suit of armor.
I originally tried a +10 to dc for faster crafting like the book rules use. I abandoned it early on because the craft times became too fast, even with relatively low skilled craftsmen. The double dc method was the most reliable one I've found so far.
So if you want to make alchemist fire's faster, you need a +30 craft skill, so its very hard to do. You can use assistant's of course to make this process faster.
If you feel that alchemical items are too slow in this system, I would make the following change: alchemical items do not cost x2 even when their craft dc is above 15.
Hope that helps