I note my fit and record all measuements i need to recreate that fit if the bike is destroyed or stolen.

I have a spreadsheet which I use to compare frames. For example my specialized has a 17 mm longer horizontal top tube than my colnago but that doesnt mean the reach is 17 mm longer because angles and HT length are different. the reach is actually 8.8mm longer

Therefore my spreadsheet allows me to enter horizontal top tube length, seat tube length and angle, head tube length and angle, stem length and angle and bar reach, number of spacers and headset size, bar drop.

This allows me calculate very easily for a given frame, what bar and stem and spacers i will need to acheive the same front end position as my "known" good fitting bike in terms of reach and height. Thats reach, not distance to saddle which will depend on the saddle.

A lot of people think that a 17mm shorter top tube will result in you needing a 17mm longer stem to get the same fit but if you work it out that is not correct. I was able to walk in to the shop and tell them the frame size i wanted the stem length and number of spacers for a frame I had never ridden and know that it was right. When I got home and measured it in real life, it was exactly the same as my other bike.

It was a very useful tool when buying a bike. I was also able to show that no trek will fit me as good a my colnago as their TT are longer and the result is I end up with more spacers and a short stem with too much drop or just a very short stem. I proved this with a test ride of the closest fitting frame. It was fine but I wanted a 90mm stem as predicted