What are Calibrated Sandstone Flagstones paving

Whats best calibrated or non Calibrated stone paving 

what are Calibrated Indian sandstone flagstone paving, Basically they are the same stone as the Non calibrated but are generally much thinner on average 22mm by having the back of the flagstone shaved off to give a consistent    thickness through out the pack . They are far easier to use and most landscapers will use nothing but calibrated stone nowadays. They came about as shipping costs increased and importers wanted and needed to get more stone in the container to keep costs down. Where as you can get 21 sq mts in a crate of calibrated you are looking at about 14-15 sq mts of non calibrated stone in a crate. Also the landscapers  like them as they do exactly the same job as the thicker slabs but are far easier to move and can be done a lot of the time with 1 person. They do work out more expensive that the Non Calibrated ones but you should be able to negotiate a far better laying cost as they can be put down much quicker saving time .