I don't know about A % but, You can set a time limit so it drops it after x amount of minutes.

First in a terminal

sync && echo 3 | sudo tee /proc/sys/vm/drop_caches

To clear current caches.

Make it a cron-job Press Alt-F2, type gksudo gedit /etc/crontab, Then Add this line near the bottom.

 */15 *    * * *   root    sync && echo 3 > /proc/sys/vm/drop_caches

**This cleans every 15 minutes. You can set to 1 or 5 minutes if you really want to by changing the first parameter to * or /5 instead of /15

To see your free RAM, excepting cache:

free -m | sed -n -e '3p' | grep -Po "\d+$

I don't know about A % but, You can set a time limit so it drops it after x amount of minutes.

First in a terminal

sync && echo 3 | sudo tee /proc/sys/vm/drop_caches

To clear current caches.

Make it a cron-job Press Alt-F2, type gksudo gedit /etc/crontab, Then Add this line near the bottom.

 */15 *    * * *   root    sync && echo 3 > /proc/sys/vm/drop_caches

**This cleans every 15 minutes. You can set to 1 or 5 minutes if you really want to by changing the first parameter to * or /5 instead of /15

To see your free RAM, excepting cache:

free -m | sed -n -e '3p' | grep -Po "\d+$"