Sorry Fuzzy, was watching a bit telly there.
The boiler is rated at 60000btu so you convert it to kw by dividing by 3414 = 17.58kw
As it is an old boiler, unless it states the output is net on the data plate, assume it is gross so divide 17.58 by 1.1 = 15.98 rounded up is 16kw.
If the cupboard (compartment) is vented direct from outside you base you vent sizes on the FULL input of the boiler
Vent sizes would be 16 x 5cm = 80cm² at high level and 16 x 10cm = 160cm² at low level
If however the compartment is vented to the kitchen and then to outside you need to calculate the compartment vent based on overall input which would be 16 x 10cm = 160cm² for high level and 16 x 20cm = 320cm² for low level.
You then need to calculate the combustion air requirement from outside so deduct 7kw from the 16kw for adventitious air = 9kw x 5cm = 45cm² needed
Easy innit. Try to remember that one as you will need it.