amavis set nice level on ubuntu 18

[webmaster at vbox.com.au]

Hello, I want to set Amavis to a nice level around 15 so other services 
take priority.  I have done a lot of looking and found where the file is 
that can accept a nice level, say Nice=15.

However that file amavis.service is in  /run/systemd/generator.late and 
is  "Automatically generated by systemd-sysv-generator"  Any changes 
made to the file are overwritten.

I've tried putting the file amavis.service in /etc/systemd/system and 
/lib/systemd/system but systemd-sysv-generator or Amavis doesn't like 
that and throws an error.

I had found the directive DefaultLimitNICE=+15  and changed both files 
/etc/systemd/user.conf and /etc/systemd/system.conf.  Still no change 
after restart the service or even a reboot.

Can do renice but it isn't persistent.

Does anyone have any pointer on where to put the Nice=15 directive so 
that it will work?

Thanks in advance, Greg.