A calculator can preserve entered decimals exactly without making the physical measurement exact. SlopeCalc exposes where rounding enters and keeps downhill values negative.
Use a consistent direction
Positive rise means uphill in the chosen travel direction; negative means downhill. Reversing direction reverses the sign but not the magnitude.
Exact inputs do not imply exact angles
Decimal strings and defined unit factors are stored as reduced rational numbers. Arctangent and tangent use finite floating-point approximations and carry an ≈ marker.
A slope number is not a design approval
Permissible grades depend on jurisdiction, purpose, materials, drainage, traction, accessibility, and tolerances. Check authoritative requirements and qualified professionals separately.
Report precision with the result
Keep exact fractions or terminating decimals for linear conversions, and label rounded angles with an approximation mark and chosen decimal places. Also retain the original measurements, because more displayed digits cannot recover accuracy that was absent at measurement time.