Oh No Not another Fuel Mix Question!

Woody,
Get creative. How far away is your closest drinking establishment? Or, as we call them in the USA, a bar. Ask the proprietor for cast-off bottles with caps...vodka, whiskey, rum...anything. They can be used to mix and store your fuel. I have cut plastic milk jug bottles to make new seals for bottle caps like that.
Even plastic rubbing alcohol bottles can be used for the same purposes.

The Andy Batts calculator above is used for mixing two fuels in quarts, that is 32 ounces. I reverse-engineered it for myself so that I mix eight ounces at a time. My measuring cup is actually a drink mixer 2 ounce cup in 1/4 ounce graduations. So, I simply use two ounces of my "high test" plus two ounces of my "high test" mixed with 2 ounces of my "low test plus 1.5 ounces of my low test, and add 1/2 ounce of castor. I would mix as larger measures if my measuring cup were bigger.

Use your 10% N fuel as one of the fuels, and your 100% methanol as the second fuel.  Practice with it BEFORE actually blending your resultant fuel.
Don't worry about the difference between imperial vs metric...just consider the ratios themselves. As an example, I would substitute each imperial ounce and mix using 10 ml in place of each ounce...such that you would have a final mix of 320 ml, or just less than a standard 375 ml bottle of booze (if that is a standard bottle where you are). Or mix as each ounce is commonly roughly 30 ml...then your blended lot would be just under a full liter.

Since you know your fuel is 15% oil, but don't know the ratio of synthetic to castor, just call it 7.5%/7.5% in the calculator. Even if you call it 100% synthetic, you'll see the castor content would be high enough to protect a Cox engine. Check out the calculator with some trial runs. I think it will work for you.

Since you will be low on the nitro,.you may find the engine may run best with a thinner head gasket, or even without the head gasket.

OldEngineRod's fuel company link also shows Nitromethane by the liter...lucky you guys!

Play with that fuel calculator to see what happens with the second fuel as 75% methanol and 25% nitro...

I strongly recommend storing all of your ingredients in air/liquid tight containers. The fuel jugs for petrol/diesel aren't air/liquid tight to any significant degree. I wouldn't carry my fuel in them for any flying. My own fuel jug is a six ounce flask. My mixing jug is a one quart old school heavy brown glass medicine bottle. If/when I go fly the big stuff, I have a one quart fuel jug for that.

Most of the top-shelf guys DO advocate measure by weight. I'm not top-shelf by any stretch, so I'm completely satisfied using fluid measure.

Synthetic oil is good and aids keeping the castor build-up down. Castor is required for the piston ball-joints. So as long as I keep at around half and half synthetic to castor, I'm comfortable with that.

If I were to tilt it either way by any significance, then I'd be mixing for particular engines. More castor for my older technology engines, more synthetic for my newer engines.

OS, Norvels, AP .061s, and a couple of OS knock-offs are lumped together as newer engines. Enyas, Fox and Cox (and one un-tested Testors 0.40 throttled engine) are my older engines.

I consider 2/3 synth to /1/3 castor a fair mix for the newer stuff, and then flip flop for the older ones...and those are the extreme limits, not targets for me.

I generally just try for half and half for both sets. Then, with respect to total oil content, I shoot for around 20% all around, but I'll go heavier (up to 25%) for the really old stuff, and wouldn't cry about getting down around 15% as a minimum for the newer ones. Again, a few points either way haven't broken anything yet. Maybe I need to run the stuff more to the point of failure.

With respect to nitro, I try to keep around 20% all around. I'll take my regular brew and de-tune it down closer to 10% for the Fox. But I need to watch that one carefully as I have only a few quarts of bench time with it.

I also don't feel like a few percentage points are going to make enough of a difference in the Roundie-Roundie patterns I fly. None of my engines have suffered for it.