Varignon's Theorem, also often called the principle of moments, is a very useful tool in scalar moment calculations. In cases where the perpendicular distance is hard to determine, Varignon's Theorem offers an alternative to finding that distance.
In it's basic form, Varignon's states that if we have two or more concurrent forces, the sum of the moments that each force creates about a single point will be equal to the moment created by the sum of those forces about the same point.
On it's surface this doesn't seem that useful, but in practice we will often use this Theorem in reverse by breaking down a force into components (the components being a set of concurrent forces). We can solve for the moment exerted by each component (where perpendicular distance d is easier to find) and then simply add together the moments from each component to find the moment from the original force.