How can you calculate ring strain in beta lactams for different penicillin drugs?
For the calculation of ring strain for ring-opening metathesis polymerization, I used DFT calculation.
In generell you open your molecule at the position you want to and saturate the bonds. Then you compare the enthalpy of products and educts.
As an example you can look at cyclopentene.If it is opened during Metathesis and you saturate the doublebonds at the end of the chain with two $\ce{CH2}$ or one ethene molecule.

This has also been reported in literature for example:
https://onlinelibrary.wiley.com/doi/full/10.1002/pola.28695
I have also done a quick search and in this paper they have used the same technique (they just used ethane instead of ethene because they saturated single bonds instead of double bonds). They used post Hatree Fock B3LYP to calculate the enthalpy. I think you are fine to use either one.
https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cphc.201900173