Do Slater's rules apply to neutral lithium (at least approximately)?
You need to use the factor of 0.85 for each of the $\ce{1s}$ electrons:
$$Z_{eff} = 3 - (2\times 0.85) = 1.3$$
as per rule #2 in the Wikipedia article:


  
  If the group is of the [s p] type, an amount of 0.85 from each electron with principal quantum number n one less than that of the group, and an amount of 1.00 for each electron with principal quantum number two or more less.
  


The $\ce{1s}$ electrons both qualify as having "a principal quantum number $n$ one less than that of the group" (here, the principal quantum number of the group of the $\ce{2s}$ electron is 2). Here is a nicely worked example for further edification.