Scaling vs. Flat CDR runes

I recently picked up and have been running standard AD runes (AD Marks, AS quints, armor yellows, MR blues) but after doing a lot of research online found that CDR blues are usually recommended for him. I can see why that is, as it helps tremendously with his early clear.

My question, however, is I cannot find any posts online that are < a year+ old that talk about this. After doing my calculations, the scaling CDR (- .09%/lvl) outscale the flat CDR (- .83%) runes at level 10 (level 9 is virtually the same amount as flat CDR.) Does this make it worth the extra cost (820 vs. 410 IP) to buy the flat CDR, as the scaling take effect less early than other scaling runes? Or is it worth it more in the long run to have the 15% @ level 18?