Artificer Feats Help

[RaistlinSees]

So I am playing in one of my friends campaigns as an Artificer starting off at level 11.

So my biggest questions are for the following feats;

Extraordinary Artisan
Legendary Artisan

So I have read the Complete Cost Reduction Handbook (http://brilliantgameologists.com/boards/index.php?topic=7274.0) but am still not very sure on what the calculations would be.

For Extraordinary Artisan do I just multiply by .25 by the cost to craft an item. I always figured it to be Market Price/2 then you would get 25% of that to have how much your crafting cost would actually be after feat.

For example , the market price for a Type IV Bag of Holding is 10,000gp. So normally I would pay 5,000 gp in material cost right? Would it be now at this point that I would add my feat of 25% off so to say and pay 1250? I feel kind of lost and I want to play the crafting aspect of the class correctly.

I see that on the Reduction Guide it has
Extraordinary Artisan feat (ECS) Reduced to 75%

Can someone help me on this please? I would really appreciate it

EDIT: So I saw this cost = [market - (market x 0.5)] - (market x 0.75) = market x 37.5%
on the Reduction Forum's post. How did he get 37.5?