So I think I made some headway, thanks to Allen Browne again http://allenbrowne.com/ser-58.html
I restructured tblNIS
SELECT RT.EffDate, Nz((SELECT MIN(RT1.EffDate)
FROM tblNIS RT1
WHERE RT1.EffDate> RT.EffDate)-1,#12/31/9999#) AS EndDate, RT.WRange AS LowerWK, (SELECT MIN(RT1.WRange)...