I have never been able to tune a rig accurately “by ear”. I always use a loos tension gauge, or the stretch method with the meter stick. But... that only gets you so far.
Taking the boat out sailing in a good breeze is a very important part of the process. Skipping that step, out of economic requirements, is why so many “Professional” riggers come up short in tuning rigs, and not just Amels.
Olivier has written an excellent summary on tuning that is here in the archives. The is no reason you can’t slack all the rigging and start as if your mast was newly stepped.
If your back stay was too loose, then your forestay must be too, assuming the mast is straight, since they pull in opposition to each other.
Norfolk, VA, USA