1100 -- to get R value 2.8 walls and 4.0 celling ( Is it worth to spend 1100 for small increase in R value ?)
If you want to base that answer on data you can use the BRANZ calculator here : https://www.branz.co.nz/energy-efficiency/nzs-42182009-calculation-method-tool/
You'll be able to enter all your wall and ceiling data and so on and see the difference between different insulation (and window) specifications.
Then you'll have to weigh it up against any energy savings over the life of the building (or your ownership of it if you prefer) as well as the improvement in comfort, if any.
Without knowing the buildings details for example, the ratio of glazed to wall area, you're guessing.