I joined the seachem forums under the same name I have here and asked a similar question;
Pruigen and Matrix in a Aqua-Tech 30-60 Filter - Seachem Support Forums
If you look at their 8-23-13 response, you have it exactly right.
A tank will not "OD" on Purigen at least within reason. 100 ml works fine in my 29 & 38 gallons and I'm planning on adding a second one to the 38 gallon.
On the other hand, decreasing it probably won't hurt you, if you read their website on how it works, it will simply be depleted faster. I'd stick with two or make it 3. I'd also stagger the replacement dates.
As far as rejuvenating them in bleach like they claim you can do. I am too scared of bleach to do that ....... but most people I believe do at least 1 rejuvenation per bag.