Diamonds clarification help please

I need to find out if there is a publication which would explain in
detail requirements for diamonds. I understand, after reading some of
my books that you need altitude, goal and distance.
O'K altitude is not a problem but...what do I use to prove my altitude?
Do I need a barograph or can the Volkslogger data be used for that
purpose?
Goal...I fly out and return flight totaling of 186 miles (300 km) or a
triangle equal to 186 miles (300 km). Is this correct?
Now the main issue..my distance...do I have to fly out for 310 miles
(500 km) land somewhere or can I fly out and return flight with combine
distance of 310 miles (500 km)? Can anyone explain to me this issue?
Thanks for your help.