I favour 1/4 inch open for a cold start......
As I explained to a young lady student "¼ in means just that. You
might have been told by some that an inch is perhaps rather different to its true size, but in this case boasting is irrelevant - ¼" means ¼"!".............
Bluntie, at the cranking speed of a 'dog, fatter sparks from using both magnetos is probably of more benefit than an impulse magneto alone. But what do the FRCs say?