Offset just one column in table that has 2 columns?

I have a table with 2 columns that I will be joining to another in a query. I won't paste/explain everything going on with that join as it is a mess in progress.
In the first column is months 1-12, and in R78, there is corresponding values by month.
The join on month helps me take sales number * R78 corresponding row of month to give me a value.
Example would be if sales number is $4000 and sale came through in March for a long time employee, then it would be $4000 x 6. Issue I am having is that if employee started in April and their sales April was say $2000, I need to times that by the first month, which is 1.
I need a way to do this without hard coding any dates so that this query wont have to be updated in the future. I have the date part figured out but I need to figure out how to create that offset.

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MONTH	R78
1	    1
2	    3
3	    6
4	   10
5	   15
6	   21
7	   28
8	   36
9	   45
10	   55
11	   66
12	   78

Basically what I am looking for is if I want month to start at 4, I want R78 to start over at value 1.

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MONTH	R78
4	    1
5	    3
6	    6
7	   10
8	   15
9	   21
10	   28
11	   36
12	   45

I tried various order by, offset, fetch queries but I can't get to where I would like. Datatypes are both smallint.
Examples tried:

SELECT *
FROM dbo.RT_R78
ORDER BY month
OFFSET (select count(*) from dbo.RT_R78) - 11 rows

SELECT *
FROM dbo.RT_R78
ORDER BY month
OFFSET 1 ROWS
FETCH NEXT 11 ROWS ONLY

This is a method that uses the window function sum() to help reconstruct the dataset starting from the desired month :

with cte as (
  select *, sum(case when month >= 4 then 1 else 0 end) over(order by month) as rn
  from RT_R78
)
select c.month, r.R78
from cte c
inner join RT_R78 r on r.month = c.rn

Result :

Demo here

What if the employee starts in other months?
Here is my solution (as I understood the question):

WITH RT_R78 AS ( -- give me a table with the R78-Value for the 12 Month
SELECT * 
FROM (VALUES (1, 1), (2, 3), (3, 6), (4, 10), (5, 15), (6, 21)
           , (7, 28), (8, 36), (9, 45), (10, 55), (11, 66), (12, 78) ) AS t (MonthNumber, R78)
)
, Employees AS ( -- give me a table with 3 Employee and their StartMonthNumber
SELECT * 
FROM (VALUES ('Employee1', 3), ('Employee2', 4), ('Employee3', 9) ) AS t (EmployeeName, StartMonthNumber)
)
SELECT Employees.EmployeeName, Employees.StartMonthNumber, RT_R78.MonthNumber, RT_R78.R78
FROM Employees
    INNER JOIN RT_R78 ON RT_R78.MonthNumber<=13 - Employees.StartMonthNumber
ORDER BY Employees.EmployeeName, RT_R78.MonthNumber

https://dbfiddle.uk/EaxfpWyi