There is no numerical input for the curvature in the Inkscape GUI. This isn't something you will typically find in vector software anyway, not just in Inkscape. I mean you could technically edit the path data manually in the XML editor, but that's not exactly user friendly.
I'd suggest a different approach.
Drop a vertical guide, and draw one half of the shield as a closed shape directly against the guide, use snapping to get it exact, then apply a Mirror Symmetry Live Path Effect.
If you need to convert the path effect to an actual shape, do Path > Object to Path, then do Path > Union