Franchi,
If i'm unsure of the remaining ether content of my fuel, I use the method outlined by atesus, a day or so does the tricI like to use 100ml in a graduated vessel, to make the maths easier
I generally use 2 Fuel mixtures, for ball raced engines I use what is commonly known in Aus and UK as the Olly brew, being the mixture recommended for use in Oliver engines. That mix is 20% Castor Oil, 30%Ether and 50% Kero, to which DII is added in whatever weather conditions and intended useage determine.
For plain bearing engines I use 25% Castor Oil. 30% Ether and 45% Kero, 1.5% DII is added after mixing.
I NEVER use an electric starter on any of my diesels. They generally start on the last running setting, but may need a bit of running to warm up.