There appears to be a list of items that you cannot get back from desynthing. These include dissolvents, glues (terminus putty doesn't count, putties are apparently more salvageable than glue), oils, and waters. I can't remember if you can salvage varnish or not. Most food items are also unavailable, but almost no food is salvageable, so that's generally a moot point. I think fish are also irretrievable, but I could be wrong, I don't desynth stuffed fish, like, ever. I should make a list at some point... This is something I only noticed recently.
The point of the spreadsheet is so people don't have to plug in numbers. You just enter your skill and class, and it spits out what the "best" things to desynth are, assuming I'm right about a bunch of things (I'm at least kinda close! I mean stuff like the exp formula and the yield formula, which are unclear, but I think I've figured out roughly). The problem with this approach is that it uses only mat values from my server.
I think, once the database has been filled out more, I'll throw in a page for cost overrides. It'll show a list of all the materials and the price on Faerie next to them, and you can put in a different price as an override for the leveling sheet if you want to. That should be a good mix of server-bound numbers and flexible numbers. Of course, anyone can copy the sheet and plug in all their own numbers, but I don't recommend it as you won't get my updates to the database/formulas/features.
Dragon Fangs are worth 20 gil on my server, but the other items aren't bad. 12k isn't a bad price for it for the exp, but you won't make 12k back on average given you have an estimated 53% chance of getting junk, and the other items are worth like 4-10k, and you can only get one back. Also, those are only r139 (c56) despite requiring level 57 to equip. Griffin Talon Needles, btw, are 15k each and r145. Depending on your dSkill, those might be better.
If Hardsilver Ingots are cheapish, you may be better off for pure leveling with Hardsilver Earrings of Gathering. They're r142 and only take 9 crystals, 2 Hardsilver Ingots, and 1 Hallowed Water. Since you can't get the water back, if my formula is correct (and I'm not saying it is), you should get worthless demimateria 33% of the time, 1 Hardsilver Ingot 33% of the time, and 2 Hardsilver Ingots 33% of the time. That means you get back half the Hardsilver Ingots you spend on average and some of the crystals (1-2 of each). The Hallowed Water is made out of one item that should be super cheap (the 2x abalathian spring water) and 2x abalathian mistletoe, which you can buy from a vendor for a total of 6.7k. So, it should cost 7.5k per synth in addition to the hardsilver. Hmm.... thinking about that though, they're probably more expensive than Griffin Talon Needles, and enough more expensive that you should just use the Griffin Talon Needles instead at that level.
So... I really don't know. I'm not that far yet to try it myself. I'm GSM dSkill 145, but I don't have access to the last two vendors yet since I'm slacking off on my DoW/DoM classes xD (my highest class is currently FSH).