You need "Single Positioning"

In reply to Contextual kerning (LookupType 8, chaining contextual positioning):

You need "Single Positioning" used in your kerninglookup:

lookup kernHorizontalKerninglookup2 {
pos \f \space' \T ;
} kernHorizontalKerninglookup2;

lookup SinglePositioninglookup3 {
pos \space <0 0 300 0 >;
} SinglePositioninglookup3;

if I understand it correctly...

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