chain tension noob

Ok, I decided to check the chain tension on my gpx250 and the manual says there should be movement of between 35 and 45mm. When I pull the chain down to the ground it moved about 5mm but when I push it up it moves roughly 50mm. How do I check the chain to see if it has the correct tension? Am I checking it correctly? I bought the bike ~3 weeks ago from a dealer (its 2nd hand) and rode it for the first time yesterday. Should this be something I should get the dealer to tightten for me or is it a regular maintainence thing I should learn to do myself?