Looks good.  Marking as accepted.

Before applying, consider adding back the part of the docs with the '1 +
floor(...' definition.  I think it provides a useful alternative way to
look at what the method does.  Also, it gives a useful mathematical
expression that can be used in reasoning about invariants.  More
importantly, we should provide it because it is so easy to make a
mistake when rolling your own version of the formula (i.e. using ceil
instead of floor or having an off by one error).