  The method suggested in this paper allows to express the n-th order
renorm-group equation solutions over the powers of the two-loop solution, that
can be obtained explicitly in terms of the Lambert function. On the one hand
this expansion helps to get more reliable theoretical predictions, on the other
hand the scheme dependence problem can be understood better. When using this
method, Stevenson scheme invariant expressions can be obtained easily, the
scheme dependence emerging from the perturbative series truncation can be
estimated and reduced. The `optimal' choice of the scale parameter allows to
have at the three-loop level the scheme dependence magnitude corresponding to
the four-loop level etc. The new criterion, principally different from the
Raczka criterion, is introduced.
