checking filesystem free space

Hello
I would check against free space, not against used one.
EG, consider this scenario;
df -h | grep opt
/dev/mapper/centos-opt   313G  290G  7.9G  98% /opt

for this server monit  is reporting 92% used, but I have only 2% free.
I would receive an alarm when free space is under a certain  treshold.
Under 5% it should send alarm., for example.
How can I get it?
Best regards

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