Please see the tutorial
Flexible option is widely used in assembly by many designers. The concept is very simple please follow the tutorial .
Make 2 circular plates and assemble it , using the dimension shown in the image
create a spacer as per the dimesions shown in the image using extrude command
right click the extrude feature in the model tree, and select dim 20 (length) and again right click to go to properties and change the dim name to "length" as shown in the image
go to file>properties and in properties window select change against flexible as shown in the picture
select the dimension which you want to make flexible as shown in the picture. save the part
Go to assembly and call this part for assembly, click "yes" in the message box as shown in the picture
In the box go to methods and select distance as shown in the picture, and then select the inner face of the assembled circular plate, you could see now that the length of the spacer is changed, and is equal to the distance between the plates
you need to assemble the spacer by mating and aligning the axis as shown in the picture
Now if you change the distance between the circular plates the length of the spacer automatically get adjusted.