Crafting all kinds of Arrows (Pathfinder)

OK, just make some plain stupit calculations that turn out to be a pain in the ass because I'm so unfamiliar with the crafting rules. So we want to craft arrows (because we found some Mithral speer heads and I got curious).

The rules say

1. Find the item's price in silver pieces (1 gp = 10 sp).
2. Find the item's DC from Table: Craft Skills.
3. Pay 1/3 of the item's price for the raw material cost.
4. Make an appropriate Craft check representing one week's worth of work. If the check succeeds, multiply your check result by the DC. If the result × the DC equals the price of the item in sp, then you have completed the item. (If the result × the DC equals double or triple the price of the item in silver pieces, then you've completed the task in one-half or one-third of the time. Other multiples of the DC reduce the time in the same manner.) If the result × the DC doesn't equal the price, then it represents the progress you've made this week. Record the result and make a new Craft check for the next week. Each week, you make more progress until your total reaches the price of the item in silver pieces.

So

1. price for 20 arrows is 1gp (10sp)
   1. special materials
      silver, alchemistic (dmg -1): +2gp
      cold iron: x2 (→ 1gp)
      Adermantine (count as masterworked, att+1): 60gp
      Mithral: +500gp/1lbs → +1500gp
2. DC for arrows is 12
   to build faster: DC +10
3. pay 1/3 or the price
4. Take10 + Skill + Tools
   Work per day: DC x Skill Roll / 7

This sums up to

• normal arrows (pay 4sp/20 arrows)
  10sp/20 arrows; DC 12 +10 ⇒ DC 22; 22×30/7=94 sp work per day ⇒ 94/10(round down)x10 arrows = 180 arrows / day
• cold iron arrows (pay 7sp/20 arrows)
  20sp/20 arrows; DC 12 +10 ⇒ DC 22; 22×30/7=94 sp work per day ⇒ 94/20(round down)x10 arrows = 80 arrows / day
• alchemistic silver arrows (pay 10sp/20 arrows)
  30sp/20 arrows; DC 12 +10 ⇒ DC 22; 22×30/7=94sp work per day ⇒ 94/30(round down)x10 arrows = 60 arrows / day
• adermantine arrows (pay 204sp/20 arrows)
  610sp/20 arrows; DC 12 +10 ⇒ DC 22; 22×30=660sp work per week ⇒ 7 days / 20 arrows
• mithral arrows (pay 5004sp/20 arrows)
  1501sp/20 arrows; DC 12 +10 ⇒ DC 22; 22×30=660sp work per week ⇒ 16 days / 20 arrows

Calculated with: 16 skill ranks, masterworked craftsman bonus (+2), master worked tool set (+2) and Take10.