What would the recommended minimum and maximum percentage full for starters in Erlenmeyer flasks be? What about for boiling wort in them without boilover?
Don't get anything smaller than 2L. I was cheap and bought a one liter at first. Boiling in it is a babysitting session. I have a 2L now and I boil 1800mL in it no problem. Most yeast manufacturers like for you to shoot for 2L starters. If you worry about boilovers more than $$ grab a 3 or 4 liter.
A single 2L starter works fine with ales. When I do lagers, depending on viability, two to three successive 2L starters is what I end up with.