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The larger the ET the closer to the centreline of the vehicle the hub face is.
Not really. The distance from the centreline of the vehicle to the hub face has nothing to do with ET. That's determined by the suspension and hub geometry. You can put any ET wheel on it you like, that number isn't going to change.
The ET is the distance in millimetres between the centreline of the wheel and the hub mounting face. In unusual circumstances it can be negative (where the mounting face is closer to the inside face of the wheel rather than the outside face) but in the vast majority of cases ET will be positive, as the brakes and hub will sit inside the wheel.
So an ET25 wheel plus a 10mm spacer should be equivalent to an ET35 wheel.
Definitely not.
A wheel with a 35mm ET has a 35mm gap between the centreline of the wheel and the mounting face. A wheel with a 25mm ET has a 25mm gap, so assuming everything else is equal the wheel will stick out 10mm further. To turn a 25mm ET wheel into a 35mm ET wheel, you'd have to machine material away from the mounting face of the wheel, so adding a spacer is the opposite of what you want to do.
A 10mm spacer would turn an ET-25 wheel into an ET-35 wheel (both with negative ET values) but as noted above negative ET values are unusual. I run ET-25 wheels on the back of my race car, and they were special order due to -ve ET wheels not being available (in the type I wanted at any rate) OTP.
All explained here.
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Brommers
Dammit
The larger the ET the closer to the centreline of the vehicle the hub face is.
So an ET25 wheel plus a 10mm spacer should be equivalent to an ET35 wheel.
All else being equal - if the wheels are different widths then you might want them to be in different (relative) positions to clear for e.g. the suspension strut.
Unless I've got this the wrong way round.