Draw/Impress default grid is 1000 for Metric and 1270 for nonMetric Subdivision is always 3 but if we can make 3 for Metric and 4 for nonMetric the grid will be similar between metric and nonMetric grid, so Gallery shapes fit always.
Some information can be found maybe in core/officecfg/registry/scheme/org/openoffice/Office/Draw.xcs
1270 are 0.5" (or 1.27cm) while 1000 result in 1cm (0.39"). If we go with 3 subdivisions for metric it result in 0.33cm steps vs. 1.27/4 is 0.32cm (instead of the current 0.5"/3 = 0.16in resp. 0.4cm). So I support the idea of metric/nonmetric subdivision defaults.