Does the plan have any documentation to justify the cost projections? It should have a schedule showing the years in which certain expenses are expected to happen and some reason for the magnitude of that expense. So, for example, some bit of equipment might need a round of routine maintenance every 5 years and the cost of that is probably known from the most recent time it was done. Reasonable assumptions about inflation can be made.
A decade or so ago we had no end of trouble because a managing agent said at an AGM and it was put in the minutes that we might need $100,000 in each of the next 5 years. It didn’t happen. It was nonsense but the consequence is that we still have people anxious about our sinking fund even though our 2015-25 plan is very well documented (even if this ex-treasurer says so himself). Our biggest non-routine expense is $30K likely once every few decades and we don’t have many of the expenses that other owners corporations do have.