I do it by eye. As for getting magic gear you have a chain ring and a casette. Wrap you chain around and bring together. Note how many 1/4" you are from joining. ever tooth you add or take away from the front or rear adds/takes away 1/4" from the length of chain needed.
So if a 32:16T is 1/2" from joing then a 34:16T would join or a 36:18" would join with a extra 1" of chain.
So if a 32:16T is 1/2" from joing then a 34:16T would join or a 36:18" would join with a extra 1" of chain.