I was going to update my previous posts on skilling up cards, but after going through charts I really realized that the amount of math I was going to have to do was just too much. Also with the cost variables of certain cards not only as a base, but at certain skill levels vs skilling them youself.
Because of this I'm going to just add another area here about the times when using skilled cards seems to be more valuable.
Personally after figuring in everything, using skilled cards is not something I would do before SSR/Legend quality cards at high skill levels for a couple reasons.
1) Skilled cards cost more to skill up yourself before using. For example, Jean-Baptiste uses the example below of a Rare feeder giving 3.5% chance to skill up an SSR from level 5 to level 6. A rare feeder with a skill of 5 gives 9.6% chance. This means using 10 skill 5 rare feeders will net an almost sure skill up. Unfortunately each rare would cost about 4HP to level to skill 5. Meaning each skill five feeder has a rough cost of 4HP x 10 of them = 40HP to move from 5-6. Using level 1 Rares has an estimated cost of 20-24HP after multiple attempts.
2) Assuming you can purchase the necessary skilled cards for smaller amounts of HP, they still require a long time to accumulate with Bazaar tickets. HR will take five days to accumulate 10 forgoing all other trading.
3) The math is just wonky and I don't wanna do it.
Now, when I WOULD use higher skilled feeders.
Levels 7, 8, and 9 of SR. SSR, Legend I would consider it. The low percentage chance from the high cost per attempt makes sense to use higher skilled feeders.
Remember that each attempt is its own unique attempt so since the chances technically aren't better each time you fail, making purchasing skilled R/HR in those situations very appealing.
Finally, thanks to all the people who let me know skilling cards before using them to enhance a skill is a viable method. Thanks to Jean-Baptiste for the link HERE that shows a chart of all the % and skill ups.