oldboy wrote:400m time x 1.1 = A

A x 1.1 = B

Expected 800m time = A + B

A long way for a short cut.

The rule is equivalent to multiplying by 2.31. It respresents a very rough and sometimes reasonable conversion factor. Some athletes will be around this factor, others won't be. For example, Juantorena's non-WR PB of 44.26s for 400m would map to an 800m time of about 1:42 - well below his actual WR for the distance. For true 400m/800m runners (runners who are about equally able at both distances), the predicted 800m time is significantly faster at the elite (male) level. As a second-order correction, you might want to add a second to the predicted 800m time.

Alternatively, you could use a much simpler conversion: multiply elite 400m times by 7/3 to get their approximate 800m counterparts. That's "double and add a third" - you can easily do it in your head without any "proper" multiply-accumulate steps, and it has the added bonus of being a much better conversion within the specific domain described (true 400m/800m, and elite).

oldboy wrote:What is Osagie’s SB? It is 103.77 exactly the same as the Rooney prediction.

What a coincidence.

They relate to completely different subjects. There is nothing remotely even coincidental here. It's like claiming that Andy Murray winning a set 6-2 on Stan Smith's 62nd birthday is a coincidence.