Use a sample wheel that you know the width and offset of then use one of the many online calculators to work out what you can get away with.
The other option would be to use a plumb bob at the guard and measure from the string to the hub mounting face with the suspension loaded to ride height and work out what you need from there.
I have a set of those in the shed that i had on my 620 think there only 15x8 and they sat flush with the guards ill have a look tomorrow and see if they have the off set on them. i have a pic some where if you want to see how they looked on it.
Thanks for that "DJZ"..
Yes a pic would be great! I got these wheels as well but they're 15x7 and I wanna go wider I am still a virgin in getting the right offset tyres etc.