I understand your use of a label, but that is a – known, and for lack of a better solution, accepted – workaround. Normally, it is natural for a label to visible at the time of placement – you would want to see the results instantly, whether they need adjusting or not.
If you are attaching labels to elements for the sole purpose of calculating values, then – at least theoretically – that label should not be a label at all. Practically of course, that’s still the only current way to achieve custom calculated values in schedules, so scheduling labels is still allowed – however counterintuitive that approach is.
The proper solution would be a dedicated calculation object (not a label), that is attached to the element and does the calculations when requested, but not necessarily having any visible symbol on plan. Something like property objects, but available for all outputs (schedules, labels, IFC, you name it). Alas that’s a topic for an unknown future version of ArchiCAD – but because we hope to focus on the correct approach sometime, we’re not planning to improve the existing, dead-end workaround.