The standard way to approach this is
calculation over partial set / same calculation over larger set
Since you haven't made it clear what context you are trying to calculate this, I will assume it's a visual along these lines:

The measure I use here is
%ItemTotal =
DIVIDE (
SUM ( Table1[count] ),
CALCULATE ( SUM ( Table1[count] ), ALLEXCEPT( Table1, Table1[item_no] ) )
)
In the numerator, you have the sum in the local filter context. For example, in that top-left cell, this means all rows that match item_no = 1 and category = "X".
In the denominator, we do the same thing except we remove all filter context except the context we say to keep (item_no) so it includes all category values.
If you're trying to calculate that 60% outside of the context of a visual, then you can explicitly define what filters you want. For example, this should work in any filter context:
X%_Item1 =
DIVIDE (
CALCULATE (
SUM ( Table1[count] ),
ALL ( Table1 ),
Table1[category] = "X",
Table1[item_no] = 1
),
CALCULATE (
SUM ( Table1[count] ),
ALL ( Table1 ),
Table1[item_no] = 1
)
)
See here and here for other ways to modify the filter context instead of ALLEXCEPT.