Table calculation filters cannot be turned into context filters.
Context filters are examined at the data source very early in the operation pipeline (aka database server). Tableau does table computations late in the operation pipeline (aka client).
So you're wondering why something that happens late on one machine can't be set to happen early on another computer by definition. The reason for this is because Tableau's table calculation functions, such as rank percentile, are based on the aggregate query results supplied by the database or data source.
If you wish to push that functionality early in the pipeline, you'll have to rely on functions from the original data source. If your database supports features like rank percentile, you may include them into the underlying query using custom sql, SQL calls, or by creating a database view.
BTW, your computed field repeats comparison tests unnecessarily.
If you define a computed field named rk as RANK PERCENTILE(SUM(BK Total)), it will be RANK PERCENTILE(SUM(BK Total)).
Then, because each test may presume the previous tests failed, your field can be created more efficiently as seen below.
if rk < .5 then
'Small'
elseif rk < .7 then
'Medium Small'
elseif rk < .9 then
'Medium'
elseif rk < .95 then
'Medium Large'
else
'Large'
end
