There's a small bug because of my (CZ) locale and I guess that it could also appear in other locales. Please forward this to responsible persons.

The following option supposes that "." is delimiter between integers and decimals. But we use "," as delimiter in CZ locale:
option cache-size `grep 'MemTotal' /proc/meminfo  | awk '{print $2 * 0.2 / 1024}' | cut -f1 -d.`MB

Better solution is:
option cache-size `echo $[ $(grep 'MemTotal' /proc/meminfo | sed 's/[^0-9]//g') / 5120 ]`MB