Presuming second focal plane reticle: a doubling of magnification halves the subtension & vice-versa: halving magnification doubles reticle subtension . . . therefore, for reticle size/spacing, multiply the 15X subtension by 0.6666666. Good shootin'! 
RG
P.S. Typically, the DATA/instruction sheet accompanying the scope, will indicate the reticle subtension - almost invariably, it is relative to the highest power/magnification. I prefer SFP & MOA type hashes - a typical 20X will be 1MOA at 20X; 1.5MOA @ 15X; 2MOA @ 10X; etc . . . power selection can be used to "refine" windage holds.
RGP.S. Typically, the DATA/instruction sheet accompanying the scope, will indicate the reticle subtension - almost invariably, it is relative to the highest power/magnification. I prefer SFP & MOA type hashes - a typical 20X will be 1MOA at 20X; 1.5MOA @ 15X; 2MOA @ 10X; etc . . . power selection can be used to "refine" windage holds.
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