I have found Lee’s “RAD” approach very useful. Particularly if you currently have a bike that “feels right” it can help figure out if and how to achieve a similar setup on another bike. It can also help identify what size can be made to work, if you’re looking at a new bike. In the past 18 months I’ve purchased two new bikes where Lee’s approach gave me the confidence that I was making a good decision (in terms of buying a bike that would let me dial in the right fit).
To @stevemz’s point that an individual’s proportions may depart from Lee’s formulas, that’s certainly true. But Lee also describes a way to test the setup of your bike empirically. Basically you put the bike between two picnic tables, supporting just the pedals. You stand on the pedals and see if you have full range of motion between fully upright (like standing climbing) and fully hinged (like descending).
In any event, well worth $25 in my opinion. Or for roughly the same money, subscribe to llbmtb.com for a month where I believe you’ll get the same bike-fitting resources plus a plethora of skills training.