Before you start tinkering with your bike, take a snapshot of your current set up.
If you make some changes but decide they’re not right for you after a couple of weeks, the vital statistics make it easy to change it back. It’s also handy if you need to borrow a bike, anything happens to yours or your fit changes over time.
Park Tools has positioning charts for road, time trial and mountain bikes. Just download, print, measure up and keep the details in a safe place.
Instructions on measuring road and time trial bikes are here, together with links to the road and time trial charts.
And the instructions for mountain bikes are here, together with a link to the mtb positioning chart.