Monorail flange bending wheel edge distance

[Ceelo124]

When checking the bottom flanges of monorails for bending you need to use a distance the wheel load acts from the edge of the flange. However most of the time when we design the beam we do not know what type of trolley/hoist system will be used. Is there a good assumption for what "a" (distance of wheel to edge of flange) value to assume that would be conservative/commonly used? Looking at various trolleys they seem to vary depending on type.