I would aim for a tax that makes red meat 7 times more expensive over time. This is based on the idea that most peopple eat meat 7 times a week and by making it 7 times more expensive it would force most people to move to a healthier once a week diet.
The money could be earmarked towards efforts to help people eat a healthy and balanced diet that is mostly plant based. Or it could be used towards investment into lab grown meat.
Of course I don't expect anyone would ever get elected in a decomracy on this platform so I suppose it'll have to remain only a thought experiment.
EDIT: I don't think I expressed myself as clearly as I should've so I'd like to offer two clarifications:
1. I wouldn't propose a 7x increase over night. Instead, much like the Australian tobaco tax, it would be gradual increase(e.g. yearly).
2. I suspect that this tax existing and some of it being invested in lab grown meat research would accelerate how quickly we could have widely available lab grown meat.