The default additive is set by your operator.
By default OPT uses +5 as Vfly equals to a minimum of Vref+5 on approach.
Vthreshold also requires to be not less then Vfly on approach, thus Vref+5 as the flare and thus some speed bleeding off during the flare has yet to be commenced.
For Landing Distance Required (LDR) calculations, which is what you may want to determine when checking a Landing Enroute capability, you require landing distance which is based on Vtouchdown, thus no +5 additive is required for this anymore. However a correction as per your FCTM must be included if an additive for current ambient conditions requires you to do so, such as gust correction.