Plain text format is best.
There are some test files at Kern King that you may find useful.
Kerning and spacing is one of the most time-consuming tasks, and one of the most difficult to get right. Months after uploading my updated Pali font, I am still tweaking the kerning pairs after coming across pairs that look too tight when editing my books. I always seem to err on the side of making them too tight.
I hope one day we will get improvements to auto-kerning.