I came up with a wheel offset idea a while back I posted on here but know one replied
looking at my bike one day ,I thought I would see how close the front and rear wheel offsets lined up in a straight line.
so I got my alloy straight edge 50mm #25mm and sat it on blocks next to the wheels.
I then checked to make sure the front wheel is true and in the centre of the forks .
then I made sure the rear sprocket was dead straight with the front sprocket using a custom straight edge I made up or you can do it my eye.
I then measured from my straight edge to the centre of the front rim and the rear of the front rim ( so the straight edge is running in a straight line with the front wheel.
Then I measured to the centre of the rear rim at the front of it ,then I measured the rear rim at the back of the centre of the rim.
The point of this was to see if the centre of the front rim lines up with the centre of the rear . cornering was my factor here.
my bike was 2mm out from front to rear .
I wasn't going to lose sleep over that
not bad for a old dirt bike , may be different for a GP bike...
so my point is a PE400 may have a 20mm offset that may suit your bike but not yours mates PE400 , so every PE400 frame and swing arm may have a small margin in being straight .
As HARDO said true it in the swing arm on the bike and centre the rear rim with the front rim...
Hope this makes scence just my views on wheels and offsets