It really depends a lot on what your base tobacco flavors are. For instance, some of the FlavourArts tobacco flavors are very strong, so they only get used at like 1% or 2% (0.3 to 0.6 ml per 30 ml).
On the other hand, I am doing some DIY using Perfumers Apprentice Tobacco Tri (something no longer available). For that flavoring, similar to when I use Seedman Commercial concentrate flavoring, I use about 12% (3.6 ml per 30 ml of final juice).
When I make tobacco juice I frequently add a bit of Perfumers Apprentice Tobacco Absolute diluted (diluted is their flavoring that comes pre-diluted in pure grain alcohol), just to add some "kick" to the rest of the tobacco juice. I only use 2 - 4 drops per 10ml of this TA diluted (about 0.2 - 0.4 ml of it per 30 ml of final liquid). TA is very strong, so I just add a little, shake and test.
If you get a copy of the scubabatdan Excel spreadsheet it gives some guidance, as do other online spreadsheets.