Calibrate with a medium to large size 123 block (a rectangular cuboid with dimensions 1x2x3) and, making note of environmental factors (if you have an enclosed printer and use filament dryer, that may not matter at all), and keep it as reference to compare to your filaments. You’ll obviously need accurate calipers, but you should have those anyway. Most slicers let you set scaling per-axis. You will probably need to change the scaling in one, two, or all three dimensions (but usually at slightly different ratios) hence the 123 block, it’s easy to see which dimension needs scaling. You can literally just print a rectangular cuboid, but infill will probably have an impact on final dimensions so I recommend making/downloading a machinists gauge block, which just adds a grid of holes, usually counter-bored to be able to remain flat when used in sets of two as squares by combining with nuts and bolts.

Awesome work. I’m grateful it wasn’t all lost. I’m so glad to be rid of that website/horrendous app. Lenny/Mlem will take some getting used to but it can’t possibly be worse than the first party reddit client (rip Apollo)