I'm having a problem making a basic modification to a font in FLS. I'm only learning FLS, so I assume I'm doing something stupid, but I can't nail down what it is.
I'm trying to modify the font Crimson, adding in a tabular oldstyle. I've already added two discretionary ligatures (no problems), and amended the OT substitution so that it correctly subs in ligatures and c2sc, and it all works great in-browser using font-feature settings (the main aim of the exercise).
I copy the oldstyle figures to a line in the private use area, set a width, name them, add a tnum1 OT class, then add a @pnum1 to @tnum1 OT substitution, then compile, at which point I get the "GPOS feature 'kern' causes overflow of offset to a subtable" fatal error. If I bypass the classes, and just substitute manually (zero by zero.tnumoldstyle &c.), the console tells me I have a script error somewhere else, despite the syntax being typed correctly throughout. It's driving me crackers, and I keep destroying perfectly good font files. Any advice?