The center of gravity must be ahead of the location on the airplane where all the aerodynamic forces operate, the neutral point (NP). This location depends on a whole herd of things, but much experience has shown that having the c.g. about 10% ahead of this point ensures a flyable airplane.
With most conventional configurations, the c.g. then is nominally located between 25% and 35%. The 25% location normally results in a very stable airplane, as the restoring moment due to its location ahead of the NP. The closer the c.g. is to the NP the more manuverable the airplane.
I don't know of any easily accessible method of computing the NP, so the "rule" is the 25-35% thing, for a normal configuration.
The NP is generally found (after some tedious work) to be around 40%.. Can be less, can be more, but that's a nominal location.
25% to 30% is probably safe for your Eindecker.