Current build is a 7' spinner. Reel is a Penn battle 4000. The diameter of the spool is roughly 50mm. Using Fuji BYAG high frame guides my test casting showed that a size 30 butt guide worked the best with 20lb. braid. Am I off base here. Some of my fellow builders thought that I could get away with a 25 guide to start. The 30 guide inside diameter is 25mm which works out perfectly for the 27x method of selecting a butt guide. Why are guides not numbered the way they are relative to the inside diameter? Build is for medium saltwater applications. Any suggestions or touts would be appreciated. Thanks

They used to be - then manufacturers started putting ceramic rings inside the older frames. So the sizing remains true to the inside of the metal frame, or from outside to outside of the ceramic ring.

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Derrek, the battle 4000 is my standard sea reel.
As i often have to use mono I sometimes go bigger than 30 if not using a match guide.
Braid 30 works fine and in Microwave the 30/8 is what i build with as it will happily do mono or braid at that size but its not a M style guide.
You could also use the 5000 reel with that guide.
Braid i would go a 25 using a Minima4 M style and that works great too but the frame height is what is important there.