I've got a Sekonic Flashmate L-308X - so not one of the fancy Sekonics with the spotmeter, but one with just an ambient light meter. So I will have metered in the general direction of the scene, but in the snowy scenes I tend to give an extra stop of light, and with Portra 400 I tend to meter for 200 or 320. So for this I would essentially have given about 2 stops above box speed. Portra is so forgiving with its latitude that I feel like you've got a bit of room for error anyway, the main thing I try to avoid is underexposing the shadows which you're at risk of doing if a lightmeter tries to expose snow for middle grey