Get a log on for Cox Engines Forum (CEF)....http://www.coxengineforum.com/
Lots of great info and original links to many of the fuel guides
Some where, I can't find now, are the COX exact formulas

All Cox TeeDee and Reed valve engines run best with 25% to 35% Nitro and 20%~25% all castor Oil.... IMO

Some of the Cox blends were 15%N and some 30%N all were 20% total OIL...usually 18% Castor and 2% Synth

Many sources------ and we (CEF members) even forced/conned hobbieco/Tower hobby to STOP selling their 1/2a fuel(at the time 17% oil mostly synth) and re-blend it properly....now a good fuel with correct oil package

Many of us have found that 20% total oil works fine and the castor to synth mix should be mostly Castor up to 50/50...I prefer Sig castor and their Klotz

I also buy and Use Torco Nitro, Methonal, UCON LB 625, Klotz, Benol, Castor and blend my own small quantities for these little engines.... From my experimenting... a Quart of Sig Champion 25%N 1/2a blend is about as cost effective as you can get

Sig sells 1/2a fuel 25%N / 35%N and castor mix in quarts evading the haz mat fee

Riche's Brew has all Castor in any Nitro level and I am a happy customer but hate his web site

  Guy's I sure do thank all of y'all for the help/info..  Again thanks...