What type of set up are you working on? I hope i didnt mis interpret your question.
Are you asking how to figure bag placement based on a lift ratio? or are you asking what is you lift ratio based on where you put your bags?
The lift ratio should be the bag height divided by the suspension height at the axle. If you are trying to find where your bags need to be placed based on a lift ratio you are trying to acheive, you can easily find that using simple geometry.
If you are wokring on a rear setup, you will need to know the vertical distance between your rear axle and bumpstop at drop height and at max height. once you know that distance, you can measure the distance from your trailing arm pivot bolt to axle and distance from trailing arm pivot bolt to bump stop. Once you have all those measurements, you basically have the three lengths of a triangle.
If you know the difference between your bags compressed height and max height, you can figure bag placement using the principle of similar triangle, go here,(
http://www.onlinemathlearning.com/si...triangles.html).
Your lift ratio will be the stroke of the bag divided by the stroke of the suspension. Then you can use this ratio and multiply it by the distance from the pivot bolt to the bumpstop. this should give you roughly the correct location to mount the bag.
Let us know more about what you are trying to do. If you will get dimensions and pics, we can help you more.