The amount of total chlorine needed to oxidize all CC is a total chlorine that is 1.5 times the CC assuming that all ammonia is combined with chlorine.
Usually, if you have free chlorine, all ammonia is combined with chlorine.
For example, a CC of 2 requires 1 ppm fc because 2 x 1.5 = 3 = 2 +1.
If you have 1 ppm free chlorine and 2 ppm combined chlorine, then you technically have enough chlorine to oxidize the nitrogen to nitrogen gas.
However, it speeds things up to add more chlorine than mathematically needed.
Sunlight/UV helps the process along by loosening the electrons from the nitrogen so that the chlorine can take them.
So, technically, it takes 0.5 x the CC to oxidize monochloramine because monochloramine has 1 active chlorine atom.
Chlorine takes three electrons from the nitrogen in ammonia. Since each chlorine atom takes 2 electrons, it takes 1.5 chlorine atoms to oxidize 1 nitrogen atom.
When measuring ammonia in units of nitrogen, the correct amount of total chlorine is 7.5 x the nitrogen. For example, 1 ppm ammonia in nitrogen units requires 7.5 ppm total chlorine (fc + cc). 1 x 7.5 = 7.5 = fc + cc.
CC is measured in units of chlorine gas. So, ammonia ppm measured as CC is reported in units of chlorine gas.
Ammonia measured in units of nitrogen will be 1/5th the weight of ammonia measured in units of chlorine gas because chlorine gas is 5 times heavier than nitrogen.
5 x 1.5 = 7.5. So, the 1.5 x or the 7.5 x is the same amount, just using different ways to measure the amount of ammonia. Ammonia in pools is usually measured as CC and reported in units of chlorine gas. Ammonia measured by an ammonia test kit is usually reported in units of ammonia-nitrogen.
The 10x rule came from the nitrogen rule of 7.5 x plus a little bit extra to speed up the reaction.
In any case, the SLAM process is the best way to deal with CCs.