To measure OAL all you need is your calipers.
To measure from the ogive you need your calipers and a bullet comparator.
Here is a link to bullet comparators. I like the Sinclair comparator.
http://www.sinclairintl.com/cgi-bin/...TCO&type=store
Sinclair has lots of things you might read about.
HEADSPACE - The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.
HEADSPACE GAGE - A device used in a firearm to determine the distance between the breech face and the chamber surface on which the cartridge seats. Also called Breeching Plug.
Below is a link to SAAMI’s Glossary which might help you with the proper terms plus the definitions also sometimes explain things.
http://www.saami.org/Glossary/index.cfm
I can tell by your questions you are new to reloading. Take your time, read a lot (like you mentioned) get several reloading manuals (I believe Hornady has some good info on headspace in their manual) use the search here and ask questions. Always better to be safe. Don’t do anything you are not sure of. Lots of people here to help and you might look here too.
http://www.thehighroad.org/forumdisplay.php?f=15
I’ve been reloading for a while and I’m no expert, but there are more things involved with seating bullets past or longer than listed OAL’s than just buying a comparator. My advice to you would be to learn how to reload by the manuals and once you’ve done that then do some research on loading longer OAL’s. Don’t get ahead of yourself. .