The meter is balanced for visible light with an average colour temperature (daylight) and will almost certainly misread when the visible light is removed with a "black" IR filter. I think you're going to have to bracket, it would make most sense to meter without the filter & work from that, I'd suggest trying -1 -2 -3 stops find out which one looks best & refine from there.

Softness - have you remembered to correct the focus by using the distance scale visible & IR indices? It may help to stop right down but getting the right focus in the first place is surely the key.