There is a lot of different views as to the best tyre pressures for the MGB.
The handbook states 21 front 24 rear but that’s for standard tyres (14″ crossplies) – also for tyre specs of 40 years ago.
Modern tyres run at higher pressures and when cars are fitted with different sized tyres to the original specs then it’s difficult to suggest the original tyre pressures still apply.
Longstone Tyres have their views…… one of many to be found on the internet and from different owners as well.
I’ve now set my tyres to 26F and 28R (now 15″ low profile radials)