Description This whitepaper presents the computational layer of Fiscal Geometry. Building on the Two-Axis Fiscal Framework—where the X-axis traces cross-border fiscal flows and the Y-axis captures intergenerational post-tax capacity—it specifies the ITI–gITI–IDI dual-index architecture for XY-based institutional analysis. The framework introduces three tightly linked indices: Institutional Tension Index (ITI): an arithmetic estimator derived from weighted combinations of representative fiscal-event series, capturing rule-side institutional tension encoded in laws, regulations, and formal designs. Geometric Institutional Tension Index (gITI): a geometry-based measure generated from density fields and projections of fiscal-event clusters on the XY fiscal plane, capturing movement-side tension arising from lived fiscal behavior. Institutional Distortion Index (IDI): a distortion measure that quantifies structural misalignment between ITI and gITI, highlighting where institutional design and observed behavior diverge within a given field. Together, the ITI–gITI–IDI system provides a coherent computational architecture for mapping institutional strain across tax systems, education finance, migration regimes, and other XY-based fiscal environments. This document focuses on the conceptual and architectural specification of the indices and is intended to complement the Fiscal Geometry: Official Language Manual (Whitepaper 2.0) by extending the framework from axiomatic language to operational measurement.