We need to make a diagnosis of your declining lung functions, if it is diagnosed already, then we need proper history and investigations as sometimes the diagnosis is wrong too. For declining lung functions, you need chest Physiotherapy exercises like deep breathing, incentive spirometry, may buy pefr device for measuring the lung functions

You didn't mention any respiratory condition. Do deep breathing excercises. Do incentive spirometry 4 to 6 cycles per day.

We need to find out the cause for declining values in pft and treat accordingly and rather than purchasing home spirometry I would advise to take pefr device for home monitoring… and practice deep breathing exercises and use incentive spirometry device to increase lung capacity

For home monitoring PEFR meter is enough

Pl , inform what is the Diagnosis made. Pl do pranayama. Pl consult.

share your older pft along with detailed history..1st need to determine the cause of your declining function, if any...then manage it accordingly