For engines to run the methanol in fuel has to vapourise to mix properly with the air so that it can burn explosively.

At cooler temperatures it does not vapourise as well. I think the point for the is about freezing point or just above. Sure someone knows this better on here.

The idea of adding nitro in cold weather is that the methanol can burn with the nitro instead, so therefore does not need to vapourise. Apparently petrol can do the trick as well, but never tried that or seen it done.

Once the engine is started of course it heats up so there is no longer a problem. The problem only exists for the first few revolutions, the temp only has to go up by a few degrees.

I've never had a problem running in cold weather with only 5% nitro, so would say no need to increase unless you run on straight fuel.