How to figure out boots Last Width?

I have an old pair of Nordica ski Boots and i'm wondering how I can figure out it's Last Width. I don't see any markings on the boot other then 260, 265 and 305mm which is the length I think. I also see marking in the middle which is letter A.

They are old school skitty Nordica GTS 6 Mens boots. I can't find info on them about the Last Width.

Is there a formula I can use to find the exact Last Width? or should i just hold the boot up to a ruler and measure it.

Thanks.