Im surprised you boiled off 50% of the starter, mine are usually in the 10-15% range for boil off. At that rate I never calculate for boil off. Maybe tone down the boil a little bit or boil in the flask with some fermcap.
With a starter on a stirplate you might never even see the krausen so I wouldnt worry too much. When you pull it off the sitrplate and let it settle check for a creamy white layer at the bottom, If you have that youre golden. Your process looks great so I dont think you have anything to worry about.