Phals can do with less light then you might expect. The recommended level is 1500 foot candles. I am growing some in very different light levels to see what results I get. I even have one on my desk 23 feet from the nearest window.
I am not concerned that you have a north window. A bigger issue would be how much light goes through the block glass. Different glass has very different light transmission levels.
nothing beats measuring the light.
Measuring light levels with an SLR camera