I know there are a few records that give Emma's name as Herrell, but I would first confirm that.
The obituary for their first born, son, Arthur Sylvester Johnston gives the parents names: Ferdinand and Emma (Hoyle) Johnston. Arthur died 10 November 1944 in Cumberland, Maryland. His obituary is reported in the Morgan Messenger of Berkeley Springs, 16 November 1944. The obituary for Ferdinand James Johnston (d. 19 February 1944) was in the Morgan Messenger, issue of 24 February 1944; it is extensive and impressive in detail, but does not mention his deceased wife, only the eight surviving children.
Harry Grossman
grossmanhj@gmail.com