MFG Fair Value, page-133

of course it is simplified but the more complex the more things that can go wrong.
the number of assumptions you need to do a discounted cash flow calculation are enormous eg imagine factoring in costs, performance fees, management fees going forward in an unstable economic environment for this stock... impossible
although Buffett has never revealed how he actually values companies. He too however, has said he likes to keep his intrinsic value calculations simple and he principally uses ROEs.
anyway, in answer to your questions relating to an ROE of 30 for '21.
It is not likely to change the ROE figure of 47 because this figure is based on an average of 9 ROEs going back 9 years
(click here to check Morning Stars ROE data)