1. Professional Workbooks: Architecture and Auditing

A professional model lives for years and passes from hand to hand. These rules save days of hunting for errors:

• Separate the input sheet (source data, parameters), the calculation sheet, and the report sheet;
• keep parameters (rates, exchange rates, tax percentages) in separate named cells, not inside formulas;
• one logic, one formula for the whole column, with no "manual" exceptions in the middle;
• auditing: Formulas › Trace Precedents / Trace Dependents, Evaluate Formula step by step, and Show Formulas (Ctrl+`);
• volatile functions (TODAY, NOW, RAND, OFFSET, INDIRECT) recalculate on every change and slow down large workbooks;
• sheet protection guards against accidental edits, but it isn't encryption. Protect confidential data with a password to open the file and with access permissions.