Как получить разницу дат в SQL и возвращаемое значение

How to get day difference from when the user registered to current date.
I have this scenario:
I have some fixed value in master table like [0,6,12,18,24,30,36,42 .....]
and suppose 
1. day difference is greater or equal than 1 and less than 6 then It should be return 1. 
2. day difference is greater than 6 and less than 12 then it should return 2 and so on.
3. day difference is greater than 12 and less than 18 then return 3.
4. day difference is greater than 18 and less than 24 then return 4.
.
.
.
So on.
I don't want to use case statements because values in master table can not be fix but value pattern will be fix. table value pattern is like that:
common difference between two consecutive values is 6 
if n=0 then 
n+1 = (0 + 6) => 6

declare @day int;
declare @regdate datetime = '2019-12-09 19:24:19.623';
declare @currentDate datetime = GETDATE();
SET @day = (SELECT DATEDIFF(day, @regdate, @currentDate) % 6 FROM tblMembers  WHERE Id = 1)
PRINT @day