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Well you have several factors to look at when you go wider. Most important is clearance on the suspension components on the inside. Second I guess would be the fit reference the outside in relation to the fender. Offset is a good number to know, but front space and rear space are also important numbers to measure if you want it to fit properly.
Also don't forget you need to design or buy the wheel based on available sizes for tires.
If you widen a wheel you change the back space and offset, but the front space will remain the same. Look up wheel offset online and you will find several diagrams to help you understand the relationship of the different dimensions.