How can I measure what is a relaxed geometry frame? I would like to know If a frame would suit me for really long distance riding (100-200km). I have read about the stack to reach ratio, which for example for one of my bikes that is comfortable for me is 1.52. But most of what I find in the road/single-speed segment is way lower. I also have a bike that is not comfortable for long rides (Polo Williambsburg) but I can not get those measurements online. Both where professionaly fitted.
But I have no been able to find a table which classified those stack to reach ratios and in some places they say that is not an indicator for anything.
How can I measure a new frame without trying it (online) to know if it is adequate for me by just looking at the geometry charts?
Stack and reach are consumer-friendly ways of doing what you're looking for, but are limited both by how widely adopted they are and to an extent how universally defined they are. A more foolproof way is to use any 2d CAD software. Model the bikes you know are working or not working for you. If you lack a record, or a record you trust, of any of the geometry, take a photo and get angle numbers that way, and just use a tape measure for lengths. Then you'll be able to model other frames/bikes and see if the position options they offer will work.
One of the tricks to doing this is answering the question of whether you can recreate your saddle setback but without having, or needing to create, an accurate model of your saddle and seatpost. Everything else is easier to express as a bunch of lines, but knowing whether you're going to be able to get the exact fore/aft position you want relative to the BB center when you're looking at bikes with different seattube angles in practice sometimes requires a measured guess of whether there's a post out there that does what you need, and then a willingness to buy that post. This comes up with Brooks a lot, where getting them far back enough can be difficult.
Use a very basic CAD software. It's easy to find one where the learning curve is steeper than necessary for this purpose. You just need some angles, lines, and circles.