[R-meta] effect size for proportion with respect to the chance expected

[Patrizio Tressoldi]

Probably it is a trivial question, but I've have some doubts on how to
calculate the effect size and corresponding standard error or variance
of the difference between a proportion and its chance expectation, e.g.
n. trials = 100; n. hits = 50; chance outcome = .25.

Thank you
Patrizio