Mapping a value to an interval

[JazonKuer]

For a scale of 5000 it will either end on 0000, 2500, 5000, or 7500.

Only replace 100 by 500 in the interval() function. However what are the valid values when the scale is 4000 ?

73874 (71500, 76500)
919227 (916500, 921500)
448634 (446000, 451000)
196040 (193500, 198500)
767177 (764500, 769500)
972937 (970500, 975500)
146383 (144000, 149000)
345814 (343500, 348500)
685618 (683000, 688000)
578875 (576500, 581500)

No that doesn't cut it either, e.g.: 269361 (267000, 272000). I would have expected that coordinate to return (267500,272500). For a scale of 4000, here are some valid intervals: (260000,264000), (262000,266000), (264000,268000).