My book on this stuff is at home - I hope I remember this correctly... You don't want caster in this case, but you do want trail.

Caster is angle of the NLG strut fore/aft.

Trail is the distance from the CL of the strut to the CL of the axle. By moving the axle aft slightly (either bend wire strut aft some at lower portion of strut, or make/use a strut whose axle is moved aft slightly), you will gain steering stability.