One of the optional rules that I universalize across my GURPS adaptations is the logarithmic Basic Lift scale discussed in "Knowing Your Own Strength", from Pyramid #3/83.  In fact, I take it a step further and apply the revised scale to all four attributes.  This condensed range makes it far easier to choose appropriate attribute levels and not get lost in granularity.

When it comes to actually selecting an appropriate Strength score for a character, again I base it on what can be proven.  Sometimes there is anecdotal evidence consistent enough to judge, such as Conan twice being described as having the strength of three men.  Other instances, especially in cartoons, can vary wildly, so I opt to measure the musculature shown in concept drawings and whatnot.