This is usually calculated as a percentage of the vertical live load and varies depending on the work being carried out between 2.5% and 30%. i.e. if you have 10 kN live load acting vertically on the platform, you might use 10% acting horizontally which would give you 1.0 kN horizontally. Please bear in mind that BS EN 12811 specifies a minimum load of 0.3 kN per bay. There is also a minimum requirement of 2.5% for nodal stability (bracing) of the standards, which also takes account of out-of-plumb (erection tolerances) and should be applied to the total load, not just the live load. BS 5975 covers these requirements better than TG20 or EN 12811. Wind loads would not normally be applied at the same time as lateral loads from live as it is unlikely that people will be working on the scaffold in the maximum wind conditions.