emulsifier and stabilizer equivalent to one yolk for frozen desserts
There is no such thing as "the equivalent" of one yolk, because yolks and other emulsifiers are not the same thing, don't work the same way, and don't have the same results. 

If you are trying to find out how much of a given emulsifier to use, either 1) use a recipe (the preferred way) or 2) find its description (for example in Lersch's book Hydrocolloids, CC lincensed) and see the range. Then work within that range. For xanthan, it is "0.25% thin running sauce, 0.7 - 1.5% thick sauces, 0.5-0.8% foams; [0.07-1%]". "Thin running sauce" is a good description of ice cream base, so you can go with that one as a starting point. You'll have to adjust by experimenting. 

Also, no idea why Modernist cuisine advises both xanthan and starch at the same time, as they are both thickeners. They must have found out through experimenting that some ratio gives an optimal mouthfeel, so take advantage of all the work they have put into it and use that ratio. It is independent of any yolk use in other recipes. If it were just because of a "thickening power" (How would you measure such a thing? Any single-dimensioned measure will be inadequate for kitchen purposes), the mixture wouldn't make any sense.