So, if I'm understanding this correctly any points earned in those 30 days will not decay?

The points will START decaying 30 days after earning them.

So, as an example, lets say I've attended three raids, each a month apart, each worth 100 points.

Feb 1st

Mar 1st

April 1st

The April one is only 14 days old, and so it will not have begun decaying yet (it's less than 30 days old)

The March 1st one is 45 days old, and so it will have begun decaying, (and would have been for 15 days). It will likely now be worth roughly 80-90 points.

The Feb 1st one is 75 days old (meaning it has been decaying for 45 days) and will likely be around half the original value, so around 50 points.

Hope that helps explain it. Each raid decays indepentendly, so your new members' earliest raids will probably begin decaying as soon as their trial period is up.

125 + 50 = 175 Total Points at the end of 100 days ?

You had everything right except the +

It should be 125 - 50 = 75.

Also to clarify, it's after 100 days after the decay starts, so a total of about 130 days.