In my aircraft, we have the advisory data for OEI case and a lot of other advisory data for non normal.
If you want to make it dispatch, it is simple criteria of adding 1,67 or 1,92 to it.
The question is do we have to do it also for the take off alternate in whatever case we can think off.
Elsa air ops specify that in the dispatch case, you have to show that you can land within the 60% of the runway for the destination and ALL alternate. so for me it includes also the take off alternate.
So I would presume that you need to show that you can land also at the take off alternate within 60% in the normal configuration, like you would do for the destination or destination alternate.
However, we know that we take a take off alternate because we can't come back at the destination due to wx. OEI is one case where (on my aircraft) you cannot come back under CAT III conditions.
I would say that we need to demonstrate that we can land at the take off alternate within 60% under normal conditions. But nothing is really written!
However no one seems to have a "real" answer to it. So let's assume, like c100driver pointed out that yes we need the dispatch criteria under normal conditions. but if we have to divert because of a non normal then it becomes an inflight calculation and as such does not require the 60% rule.
Leadsled, yes we know that we won't make the landing distance written in the books. But it is also our duty to follow the rules and make it legal.
Legality has now became a real part of our job, compare to what you might have know flying the 707 back in the old days...