Expansion/Contraction of Glycol
Okay, I prefrace my response with the fact that I have a BS and PhD in Chemistry and Material Eng.
There seems to be confusion on the expansion versus temp of glycol as the temperature changes. The coef. of thermal expansion measures the change in dimension/dimension of a material. It is a dimensionless number. for example a CvT = change in volume/volume-K or change in volme/volume per degree kelvin.
For a given composition (50:50, 70:30, 30:70, etc.) of Glycol/Water, the expansion cofficient goes down with temperature. However, at the phase change (freezing into a solid) the solid has a higher volume than the liquid, so it expands. Obviously not what we want.
If you change the amount of glycol in the mix, the thermal exp. coef. changes; more glycol, a higher expansion coeff., less glycol, lower. Also, if the temperature changes, the expans. coeff. changes. Cooler temps mean lower expansion coeff.
For your temp climate, you need to choose the lowest temp that you think will occur (and then go 20 deg colder) and mix your water/glycol mixture for it.
The statement about allowing some room for the glycol/water solution to expand, in the cited reference by k1vsk, is due to the fact that the day you fill the tank may be 40 deg F and the next day 80 deg F. If you top it off at 40, there is no room for the expanded material to go at 80. If you fill at say 80 and the temp drops to say 30, the liquid will contract and you may be lower than the amount you need to effectively work until the liquid warms up. This is why they have a overflow reservior with your engine that has a high/low fill line. The reservior allows for the expansion volume while providing enough material for proper levels if the engine is cold until it warms up.
EDIT - Thanks for all of the responses to my OP. I'll leave the Pink in.