Yes, you do need a new pin each time you break the chain - far better to get a removable link (which makes breaking the chain a breeze), there are a few different brands out on the market.

We used one of the online calculators to work out chain length - try this one:
http://www.machinehead-software.co.u...alculator.html

Or, the other way to do it is to feed the chain around the two largest cogs (including through the derailleur) and then pull the chain tight so the derailleur arm is almost all the way up.