Interesting excerpt from page 564 of the manual - note I only used a 33% mix.
50/50 Ethylene-Glycol and Water
-Is the recommended mixture, it provides protection against freezing to -37°C (-34°F). The antifreeze concentration must always be a minimum of 44 percent, year-round in all climates. If the percentage is lower, engine parts may be eroded by cavitation. Maximum protection against freezing is provided with a 68 percent antifreeze concentration, which prevents freezing down to -67.7°C (-90°F). A higher percentage will freeze at a warmer temperature. Also, a higher percentage of antifreeze can cause the engine to overheat because the specific heat of antifreeze is lower than that of water.
Looks like I might have to drain out a Litre and put in another Litre of full strength
..think..calc...think...make that 1.5L