Not a bad quick and dirty back of the envelope. Seems a little aggressive to put a multiple of 14.5x some normalized float growth figure (you used $8B) and declare thats worth $116 Billion when we have already included the value of the stock portfolio, much of which is funded by float. I do understand that growth in Float is a component of cash flow available to owners though.
I agree with you that assigning a value of $116 Billion might be to stretch a little. Buffett has for a long time, at least 10-15 years, stated that further float increases will be hard to achieve, but as a matter of fact float increased by roughly $40 Billion from 2012 to 2017. I think it will continue to increase for a long time, but maybe it would have been more proper to assign a PV of 80 Billion or so for expected future float increases. Their float is extremely enduring / long tail, so Buffett has said that it can be compared to equity.
On the other hand, maybe the multiple on the operating earnings should be higher than 14.5 given the quality and endurance of those businesses, so I still feel comfortable valuing the company at 700BUSD+.