Sounds like a good start date to me. You do get pretty cold up there. I have seedlings popping up everywhere right now. Sunrise mix is a very nice choice very pretty flowers.

I sow Impatiens in January since I find they need extra time both to germinate and grow to bedding size. Others plant as little as 8 weeks before last frost. I think warm growing conditions and better light (possibly more fertiliser) accounts for the difference in growing speed. Mine are grown in natural light and quite cool conditions.

It depends on your indoors growing temperature.

If you have a fairly warm basement (basically do not fall below 60 deg F), 8 weeks prior to planting out for accent impatiens are more than enough time. When greenhouse sown where night temperatures can go into the 50's and even 40's then a longer period would be warranted.

In any event, unless you have really cool night temperatures, then I recommend waiting until maximum 10 weeks before planting out. If this date is june 1st in your area (or even june 15th ?) then starting by march 15th would create a monster of a plant unless you can transplant to 6" pots ??

I.e. if you grow your impatiens relatively warm (above 60 deg F night temperatures) no more than 8-10 weeks MAX. If you grow your impatiens cooler then a few more weeks, but do not start them in january. These dates given on the packages are general information to cover multiple zones...