The suggestion that you find an old unwanted cheap transponder and lash it together uncalibrated to me is unacceptable.
But it isn't about your transfer flight
What are your arguments? And what is to be calibrated about a transponder?
My main point would be that a mode C is nearly useless, at least for meeting legal obligations: wherever one is mandatory, it must be mode S.
The document that LOONRAT has provided a link is an excellent resource.
There is much good work behind it, indeed. Still it is a free unchecked unverified document, and has been known to lag on changes in ruling - which are far too numerous, these days.