You should move my post (or the part talking about FC % of CYA), yours and this one to The China Shop.

Ben and I disagree about shock levels so if you are using the Best Guess CYA chart the yellow/mustard shock level would be 50% higher as you describe. However, in terms of the amount of active chlorine (hypochlorous acid) in the water, the FC as a % of CYA tracks that better than the Best Guess CYA chart and in theory would provide the same rate of algae kill for shocking. Obviously, this becomes unwieldly at high CYA levels and in practice lower FC levels will still kill algae faster than it can grow though it will probably take longer to clear the pool.

The min. FC level for inhibiting yellow/mustard algae seems to be about double that of green algae, however, so I don't think the 25 ppm at 200 ppm is going to be sufficient to kill the algae. On the other hand, if your CYA is that high, the proper prescription is to to a partial drain/refill to lower it significantly. At shock levels, increasing the FC by 50% in the chart results in roughly doubling the active chlorine concentration so your "use 50% higher shock levels" would be reasonable.

Richard