See http://forum.mikrotik.com/t/interface-data-quota/79514/1 … works well. I even use a version with 2 SIM cards in the LTE.
This is on turning down the interface when at 100%, till the period is reset this avoids getting over the limit and paying for that. But adding action at 80% should be no problem.
There was a little counting error on reset/restart/power-off of the device. Has been fixed. I only write the result to flash every 10 minutes, to limit the number of flash writes. I may loose that 10 minute consumption if reset happens right before the write operation. ISP’s count a bit different, I can go 2% higher in the quota. Quota reset is automatic on the specified day of the month.