fm-3-design

Phase 3: Design 🎨

"A well-designed model speaks before you read a single formula."

Mission

Define every visual and structural specification the model needs and hand Phase 4 (Build) a starter workbook that already obeys every convention. The output of this phase is twofold:

1. design.md — the per-model spec Phase 4 consumes (see references/design_template.md).
2. A clean starter .xlsx produced by scripts/build_template_workbook.py — already carries the 41 styles, Style Guide sheet, skeleton sheets, named ranges, freeze panes, column widths, and Heading 1 bands.

Inputs

scope.md (Phase 1) and plan.md (Phase 2).

How to use this skill

Read only the reference(s) relevant to the decision you're making. The references are intentionally narrow — single-topic — so you don't pay token cost for the rest.

When you need to decide / answer	Read
Which Cell Styles exist; their fonts, fills, borders, number formats, and tick states	references/sumproduct_styles.md
Which style goes on which cell (input vs calc vs total vs heading vs error check)	references/style_application_matrix.md
How to apply styles (two-pass pattern, IncludeNumber trap, when to call create)	references/style_creation_code.md
What column widths to set on each sheet class	references/column_widths.md
Where to put the freeze pane	references/freeze_pane_rule.md
How to lay out Heading 1 / 2 / 3 within a sheet	references/heading_staircase.md
How wide the Heading 1 band runs across the heading row	references/heading_1_band.md
How to name a Named Range (N_* constant, HL_NNN nav target)	references/named_range_convention.md
What CF rules go on error checks and inactive period columns	references/conditional_formatting.md
What Data Validation rule goes on each input type	references/data_validation.md
Which Number Format string to apply for an ad-hoc cell that no style covers	references/number_formats.md
Structure of design.md — the artefact handed to Phase 4	references/design_template.md

How to use the scripts

Goal	Run
Produce a clean starter workbook for a new model	python scripts/build_template_workbook.py --output new_model.xlsx
Refresh the reference markdowns from an updated template	python scripts/extract_from_template.py --template path/to/template.xlsm --output references/

The build script is idempotent — re-running it on the same workbook will not duplicate styles or sheets.

Process

1. Read scope.md and plan.md to understand the model's named-range needs, periodicity, and sheet list.
2. Decide deltas from the standard register — most models adopt the 41-style register and standard widths/freeze panes verbatim. Per-model deviations (a new client-specific colour, an extra period column on Cover) belong in design.md Section 1a (style deltas) or Section 8 (build notes).
3. Run scripts/build_template_workbook.py to produce the starter workbook. Verify it opens and the Style Guide sheet renders.
4. Write design.md per references/design_template.md. For sections where the standard applies unchanged, say "Standard — no deltas" rather than omitting.
5. Confirm with the user before handing to Phase 4. Present: style deltas, named-range count, validation/CF rule counts, any column-layout or freeze-pane deviations. Do not proceed to Build until confirmed.

Output

• design.md — structured per references/design_template.md (0 Header · 1 Cell Styles + deltas · 2 Named Range register · 3 Number Format registry · 4 Data Validation spec · 5 Conditional Formatting rules · 6 Column layout · 7 Navigation · 8 Build notes & exceptions).
• A clean starter workbook from scripts/build_template_workbook.py.

Phase 4 consumes design.md section by section against the starter workbook — keep the numbering intact.