The SoL percentage or "Sons of Liberty membership" in a colony in any of Sid Meier's Colonization is the percentage of the colony's population who support the idea of rebelling from the mother country. The remainder of the population are called Tories.
The percentage is based on the total number of liberty bells the colony has produced.
When it reaches 50%, productivity increases; at 100% it increases further. It can be really more than 100% (and therefore allowing some reduction while staying at effectively 100%) but shows no more than 100%.
If a colony has 10 (at the easiest difficulty; 6 at the hardest difficulty) or more colonists that are still Tories, it has inefficient government and will receive a -1 production penalty on everything produced there: food, goods, liberty bells, and crosses. This has the effect of requiring you to increase your SoL percentage if you wish to have large, productive colonies.
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In order to declare Independence, the total SoL of all your colonies must be 50% or greater.
FreeCol[]
The FreeCol method of calculating the percentage (at least up to version 0.9.5) differs from that of Col1.