Okay, that clears it up. I'm afraid you will not be able to use t1. nor @something, because Default value expressions cannot do that: Data model | Thinkwise Documentation
Query - The expression used to determine the default value. The query cannot use values from a new record.
Examples: getdate() or dateadd(month, 1, getdate())
The correct way to do what you would like to achieve is using Default logic. In here you can access all the data of the current row and you will be able to set the column value.
It will most likely be similar to this:
if @cursor_from_col_id is null
and @default_mode = 0
or @step_nr is null
begin
select top 1 @step_nr = s.step_nr + 1
from calculation_step s
where s.calculation_method_id = @calculation_method_id
order by s.step_nr desc
end
Hope this helps!