Totally agree that a 10 inch rear's optimum offset is +50. Most after market wheel companies are churning out offsets that they think might fit a wide range of vehicles, their not unfortunately specific to the commodore.
Caliper clearance is more to do with spoke design not offset - a common mistake by most people.
Do your research and get it right and don't look like one of those fools running around with the rim sticking out and the dangerously narrow tire looking like is struggling to maintain a seal.
You’re correct and that's why it’s the first thing that should be determined after choosing a style and before worrying about offsets.
There are a number of ways to get this info and of course the easiest is to see the rims you like on the same car as yours with the same brake package but this will limit your search.
The other is to speak with the manufacturer or distributor.
A local distributor should already know which rims they supply fit which applications and the good and bad of those fitments.
If you dealing direct with an overseas manufacturer then they will give you the details of their rim centres so you can make sure that they don’t foul the calliper. You may also need to help them by suppling dimensions of your setup.
This is where being handy with a tape and straight edge comes into play as you measure up your hubs, brake disks and callipers, inner and outer guards, suspension components so you can talk to the manufacturers and distributors. The measuring up is not hard as it is mainly common sense. While you do this you will also gather the info needed to assess suitable offsets.
I’ve had quite a few made to order 3 piece rims and I always prefer to measure up myself even if it’s just for peace of mind.
The Vossens the OP has chosen are a glorified cast rim and I think you’re more or less locked into whatever it is they supply as offsets. They are known to fit over the brakes on a VF2 Redline so he is halfway home on that score but like you’ve said they are not made with the Commodore in mind so whether he is happy with the look the offsets offer is a matter of choice.
Wanyebatt has a liveable setup with his Vossens that may suit the OP and without being critical if I was having them made to order I’d change the rear offsets to +50 and maybe even adjust the fronts a little.
I always see a 285/30R20 rear and a 255/35R20 front as a good grippy tyre combo on a Commodore.
And if you wish to have almost identical O/D’s then a 285/30R20 rear and a 245/35R20 front is perfect.
An 8.5 or 9 inch rim will work with the 245 or 255 and a 10 inch rim will work with 275 or 285.
