EU OPS 1.515, the one mentioning the 60% rule is not, in my opinion, restricted to planning stage. It is not like the planning minima. The operator has to ensure that pilots land within 60% of the runway. And so OMs usually include such rule, even in flight, while in normal ops.
A lot of pilots think that as long as you have two alternates you can dispatch to any destination not complying with performance requirements for dispatch. This is wrong. If the airport you are planning to fly to have two or more runways and you cannot perform a dispatch calculation satisfactorily. You are not allowed to dispatch at all towards that airport. Remember that dispatch calculations consider the mass of the aircraft at time of departure, whilst operational calculations consider the LDA vs Landing distance required. This subtle difference actually means something in terms of complying with regulations.
OPS 1.475:
(a) An operator shall ensure that the mass of the aeroplane:
1. at the start of the take-off; or, in the event of in-flight re-planning;
2. at the point from which the revised operational flight plan applies, is not greater than the mass at which the
requirements of the appropriate Subpart can be complied with for the flight to be undertaken, allowing for
expected reductions in mass as the flight proceeds, and for such fuel jettisoning as is provided for in the particular
requirement.
These regulatory requirements in Subpart F (description)/Subpart G (Requirements for Performance Class A) are valid for dispatch of an aircraft.
You may be legally allowed to dispatch towards an airport although you may not necessarily be able to land there from an operational use of the same performance data because your operator may have added margins to the performance test data provided from the manufacturer of the airplane.
In my company operating the 737 the company has in it's OM a requirement to only use "Autobrake Max" values factored by 15% for operational use as a way to determine (or more correctly help the commander determine) whether a safe landing can be made.
HazelNuts39 got it right. The regulatory agency sets forth requirements for dispatch whilst the operator needs to provide an approved method of using the manufacturers performance data for operations.
That's why you need to perform a dispatch calculation before commencing a flight to satisfy regulations, whilst airborne you need to perform an operational calculation before landing to satisfy company regulations (approved by the authority)
In other words, as a Commander in flight you need to satisfy yourself, given all relevant data available to you, you are able to perform a safe landing
Definition of safe is basically up to you/Operator
Just rememberd with regard to inflight dispatch calculation as asked for in the original post. The only time you need to perform a dispatch calculation in flight is when you have used the exemption rule (Airport having a single runway, where landing depends on a specified wind component. Dispatch can be made with two alternate airports that comply with regulations) A new dispatch calculation must be made satisfactorily in flight. However during this calculation you only need to comply with requirements for the actual runway (Not the most favorable in still wind). REF: OPS 1.515 d)