Cassette install movement

Chunky101 Posts: 108
edited June 2014 in Workshop
Fitting a set of Shimano RS61 wheels and a new cassette to the wife's bike but once fitted the cassette was a tad loose despite being done up tightly. It's a Sora 9 speed groupset with 9 speed replacement cassette.

So I fitted a spacer behind the cassette and it's now snug but I have no idea whether I've done the right thing and assume I should have taken a more scientific approach in terms of spacer measurements. The plan was always to drop the bike and new wheels off at my LBS to get the cage and chain replaced to accommodate the different cassette but was trying to save a few quid by fitting the cassette myself. Any advice on what I should have done about the cassette movement?