SQL & PL/SQL :: 70% / 30% From High And Low Priority Element

I have a priority column(possible values are 1 or 0) in a table where

i need to get 70% of high(1) and 30% of low (0) and max i can fetch for select is 50 records.

Eg1: Total if i have 60 in which 20 high and 40 low then 70% of 20 = 14 and remaining should be taken from Low i.e. 36 from low. so total will be 50 transactions.

Eg2: Total if i have 60 in which 40 high and 20 low then 70% of 40 28 + remaining should be taken from Low i.e. 22 from Low.

Eg 3: If i don't have any high then total should be picked from low vise versa.

I have below query but it is having problem when there is no low priority.

SELECT ID,PRI FROM temp tbl WHERE pri = '1' AND ROWNUM < ((70/100)*50)+1
UNION ALL SELECT * FROM temp WHERE pri = '0'
AND ROWNUM < 50-(SELECT COUNT(*) FROM temp WHERE pri = '1' AND ROWNUM < ((70/100)*50)+1)