We recently obtained partial results on the computational power of population protocols when population is assumed to be huge. We studied in particular a particular protocol that we proved to converge towards $\sqrt{\frac12}$, using weak-convergence methods for stochastic processes. In this note, we prove that this is possible to compute $\sqrt{\frac12}$ with precision $\epsilon>0$ in a time polynomial in $\frac1\epsilon$ using a number of agents polynomial in $\frac1\epsilon$, with individuals that can have only two states. This is established through a general result on approximation of stochastic differential equations by a stochastic Euler like discretization algorithm, of general interest.