At 14:26 12 December 2017, wrote:
How to calculate it?

Hmmm, complicated problem with lots of variables. Needs a
computer. How
about use the one you happen to be sitting in.

If you are operating on a grass strip, how many lights do you go by
before
wheel stop? They seem to be about 200 feet apart.

I figure if I can measure it, I should be able to improve it and adjust
as
needed. So far, so good.


with respect, I don't think there is room in the cockpit for my box and
monitor so I have always made a calculation in my head. It is a very
simple formula, find another field :-)
If you get to the point where the field you describe is the only one now
available to you, calculating if it is long enough should not be on the
list
of your priorities, and you are adopting the right approach to field
selection.