Use of Cinder Blocks

I use a piece of board (about 1" thick) under each jack. I wouldn't want to use anything too high for this purpose. Here's a reason many may not have thought of: a few years ago I noticed a fiver that originally had high blocks of wood under each leg. Well, they weren't under the legs any more when an unusually strong wind pushed the fiver off the blocks, damaging the landing gear. If the wood had not been as high it's likely the jacks wouldn't have been damaged. (Or, if the blocks had been wider that may have worked as well).