The goal for every DMARC policy is to check against 100% of all emails sent which use your domain. If you're not ready to fully adopt a strict DMARC policy, you should consider implementing a percentage policy (PCT=##) and slowly increase this value over time under each policy.
Example PCT Configurations
p=quarantine; pct=10 - 1 in 10 emails will be quarantined. The rest will be treated by the p=NONE policy.
p=quarantine; pct=50 - 1 in 2 emails will be quarantined. The rest will be treated by the p=NONE policy.
p=quarantine; pct=100 - All emails will be quarantined. You should switch to the p=reject policy and reduce the value.
p=reject; pct=10 - 1 in 10 emails will be rejected. The rest will be treated by the p=quarantine policy.
p=reject; pct=50 - 1 in 2 emails will be rejected. The rest will be treated by the p=quarantine policy.
p=reject; pct=100 - All emails will be rejected. At this stage, you should remove the pct=100 tag entirely.
Please note that the percentage tag will not work with the p=none policy.
Note, the percentage tag is entirely random. Over time, the volume of emails will equal the number you set. But it is entirely possible that the number of emails affected by the percentage tag may be much higher or lower, at random. The following formula is used to calculate this.
if (random mod 100) < pct then
selected = true
else
selected = false