manual way to check table watermark

Dear All,

We have a table which hits the watermark frequently iwth HW - Contention waits. As a workaround, we will pre-allocate space to the table. Since the table is 5TB, we only gather stats few months once. I am looking for a script which will give us some sort of estimation when it will hit the watermark without analyzing stats.

This is a normal table, without LOB or partitions.