Creeping Doom physical vulnerability

[wollstonecraft]

Can someone please tell me how CD calculates its physical vulnerability?
There are just too many numbers in the spell description for me to ever wrap my feeble brain around.
What is the max vulnerability it would inflict before reaching it's 1000 point limit?
Obviously, it would be better to have Illusion foci, but how much better?
Thanks.

-WSCraft

[BigStick]

- Inflicts 1% physical damage vulnerability per round per Illusion focus. Total vulnerability is capped at (Illusion foci * 5)%.

Here's how I interpret it -

With no focus: 0% vuln. / round
With 1 focus: 1% vuln. / round (Max = 5% reached after 5 rounds, lasts until expiration)
With 2 foci: 2% vuln. / round (Max = 10% reached after 5 rounds, lasts until expiration)
With 3 foci: 3% vuln. / round (Max = 15% reached after 5 rounds, lasts until expiration)
With 4 foci: 4% vuln. / round (Max = 20% reached after 5 rounds, lasts until expiration)