Camry Altise 2003 offset question

[Luke1]

does anyone know for sure the offset for a 2003 camry altise?

i bought my camry second hand with the original stock steel rims. There is a vibration coming through the steering wheel when I get up to about 100kmh. A front wheel alignment didn't fix it. It turns out the rear rims are bent and need replacing. genuine toyota replacement steel rims are $80 each; Ateva alloys $250 each.

i'm thinking about getting 4 cheap($100-150) alloy rims and going from stock 15" to 16" or 17".

Currently on the car is the stock rims which are 15" diameter with rim width 6". the tyres are the stock size 205 65 R15.

So I have a question about changes to the offset(I'm not sure what the current offset is) if I go to 16" or 17".

if i go 16" i can get 205 60 R16 which is less than 1% difference in rolling radius. Rim width would most likely be 7". If I do this does it mean I can keep the same offset?

if i go 17" i can get 215 50 R17 which is also less than 1% difference (only 1mm difference in diamater). Rim width would most likely be 7". Now that I would be changing the width from 205 to 215 I'm really confused about the offset. Theory tells me that to keep the balance of the wheel because I am going 10mm wider, that both the inside and outside of the tyre should be 5mm further out from what the 205mm tyre currently is - I'm not sure whether I'm correct or not in saying that tho. So does the offset remain the same? Or change by 5? Or change by 10?

from what i've read 10mm extra width probably wouldn't be noticeable anyway even if the offset didn't change but I would like to keep it all balanced if possible.

Thanks

Luke