The Goal Is to Reduce the Unknowns
You will probably never eliminate every unknown in custom construction.
That is not realistic.
The goal is to identify as many costs as possible before construction begins so that the few remaining variables do not have the ability to completely disrupt the budget.
If we know what the cabinets cost, what the flooring costs, what the countertops cost, what the plumbing fixtures cost and what the lighting costs, then we have removed a large amount of uncertainty from the project.
That gives us more room to deal with the items that truly are difficult to predict.