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There is no right or wrong baseline skip (as long as the adjustment is moderate). Just experiment. In a work of mine I use \linespread{1.05}. Why are you using \PassOptionsToPackage and not \usepackage[<options>]{kpfonts}? Note that using \RequirePackage is recommended only for classes or packages and that it has the same syntax as \usepackage. – egregAug 17 '12 at 21:20
@egreg i'm trying to write a new package for customize classicthesis, like the arsclassica package... i have read that for the mathpazo package the correct linespread is 1.05 so i have thought that for every different font exist a correct linespread – FormlessCloudAug 17 '12 at 21:28
kpfonts features an x-height (height of lowercase letters like "x") of about 4.41pt (for a font size of 10pt), which is only marginally larger than that of Computer Modern (4.31pt) and smaller that that of Palatino (4.69pt). Therefore -- and if \linespread{1.05} suits your taste for Palatino (see Why exactly is \linespread{1.05} appropriate for Palatino?) -- I suggest to try \linespread{1.01} for kpfonts (or just to leave the default settings).
\linespread{1.05}. Why are you using\PassOptionsToPackageand not\usepackage[<options>]{kpfonts}? Note that using\RequirePackageis recommended only for classes or packages and that it has the same syntax as\usepackage. – egreg Aug 17 '12 at 21:20classicthesis, like thearsclassicapackage... i have read that for themathpazopackage the correctlinespreadis 1.05 so i have thought that for every different font exist a correctlinespread– FormlessCloud Aug 17 '12 at 21:28