Rigging? What is the ratio for Spectra line strength to Mono line Strength?

[fishincrazy]

Looking to reline some of my reels and wondering if there is a specific guideline to go by when lining out a reel with spectra with a top shot of mono.
Examples:
If I want to fish with 25lb top shot of mono what strength/lbs of spectra would I use?
If I want to fish with 40lb top shot of mono what strength/lbs of spectra would I use?
Thanks
Mark

[Jighead]

For #25 mono I use #50 spectra, for #40 mono I use #65 spectra.