this is super old school topic but im on a roll and am fairly up on offsets at least with my S13 silvia! and also been doing hours of reading on amercan lexus forums for offset insights (mostly on GS300s though...)
the lower the number on the offset, the further out the wheels will protrude from the hub/wheel arch. You dont have to necessarily stay the same as factory specs, otherwise its hardly an upgrade because its simply cosmetic. The combination of width and offset are the main things to consider. if you are going wider, then the wheel will stick out Both sides of the hub e.g. go from 8" rim to 9" rim, you will have an extra 1/2" (12.5mm) either side of the hub. If you keep the exact same factory offset, your wheel will stick out 12.5mm more toward the edge of the guard, and will stick inward 12.5mm closer to the suspension. it may be that it will hit the suspension, therefore lowering the offset will bring the wheel outwards and allow it to clear the suspension, but the lower ofset will bring the whole wheel edge out more still, so you have to find the right balance.
this is a good website for working out where things will sit, but it isnt exact because how low your car is, what size tyres you run and how much camber/caster/toe you run will all effect the fitment quite a lot:
www.1010tires.com/WheelOffsetCalculator.asp