Hello colleagues

I have a an air leakage question,
The problem is when the exterior soffit vents and fascia panels are open to interior ceiling spaces, can be to a plenum ceiling or non-plenum ceiling,
these openings at the soffit and fascia are allowing air infiltration above ceiling tile,

Does anybody know what method/formula to use to calculate these air infiltration energy losses, and how they load the HVAC units ?
Any reference/articles ?

Thank you

P.S I don?t use any software, I would like to do it in excel