What is a reliable source for the caffeine content in tea?
TL;DR: (1) they are estimates based on a single sample and (2) No.

In order to pre-determine the exact amount of caffeine in a cup of tea, you would have to know many factors (as Choice Organic Teas says):


The exact tea blend and batch you're steeping;
The storage age (and conditions) of the leaves;
The exact steeping method, including times, temperatures, water source, and steps


While theoretically someone with a really generous research grant could create a giant grid that covered a lot of these factors, nobody (that I can find) has.  And the detailed reality is even more complicated than the above list, as factors like "how old was the specific tea bush the leaves came from, and was there a drought at the time" matter.