Through my travels on the internet I happened upon this cool chart to estimate how much ice is being made each night at different temperatures.  Please do not use this as a hard fast rule or make any decisions that might harm you if you are wrong, but here is the chart:

Average Air Temp Degrees F Starting Ice Thickness (inches)
Skim 2 4 6 8 10 12 14 16 18 20
30 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.0
25 0.8 0.6 0.4 0.4 0.3 0.3 0.2 0.2 0.2 0.2 0.2
20 1.4 1.0 0.8 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.3
15 2.0 1.4 1.1 0.9 0.7 0.6 0.6 0.5 0.5 0.4 0.4
10 2.6 1.8 1.4 1.1 1.0 0.8 0.7 0.6 0.6 0.5 0.5
5 3.2 2.3 1.7 1.4 1.2 1.0 0.9 0.8 0.7 0.7 0.6
0 3.8 2.7 2.1 1.7 1.4 1.2 1.1 0.9 0.8 0.8 0.7
-5 4.4 3.1 2.4 1.9 1.6 1.4 1.2 1.1 1.0 0.9 0.8
-10 5.0 3.5 2.7 2.2 1.8 1.6 1.4 1.2 1.1 1.0 0.9
-15 5.6 4.0 3.0 2.4 2.0 1.8 1.6 1.4 1.2 1.1 1.0
-20 6.2 4.4 3.3 2.7 2.3 2.0 1.7 1.5 1.4 1.3 1.2

The source of this information can be found at: http://fishingvermont.net/images/Ice/Ice_th1.jpg