  A method of evaluation of spacelike QCD observables ${\cal D}(Q^2)$ is
developed, motivated by the renormalon structure of these quantities. A related
auxiliary quantity ${\widetilde {\cal D}}(Q^2)$ is introduced, which is
renomalization scale independent only at the one-loop level, and a
large-$\beta_0$-type renormalon motivated ansatz is made for the Borel
transform of this quantity. This leads to a `dressed' Borel transform of the
considered observable ${\cal D}(Q^2)$. From there, a Neubert-type distribution
is obtained for the observable. The described method is then applied to the
massless Adler function and the related decay ratio of the $\tau$ lepton
semihadronic decays. Comparisons are then made with an evaluation method at
higher truncated orders, developed in our earlier works, which is a
renormalization scale invariant extension of the diagonal Pad\'e approximants.
