15" wheel offset

What's the best wheel offset tyre combination when running coilovers and 15" wheels with standard arches. 

depends on how wide the wheels are and the look you're after but my guess is that with your standard arches you want the wheels to sit inside the arches and not rub

so as close to standard as possible mate which is 38 I believe, I'd look for around et 33-38 to be on the safe side, could probably go lower offset than that without problems as many people do, but that depends on how low your car is on the coilies.
and you won't really want anything wider than 7j, also the narrower your wheel the lower the offset you'll be able to get away with
e.g a 7j et38 wheel will stick out the same as a 6j et25

what tyre size to go for depends on the width of the wheel, look you want and if you want the rolling radius to be the same as standard (so your speedo etc stays the same)

this may be of help:
Alloy Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

15 x 8j rear 
15x 7.5j front 
Et33 
195/45/15 good start 

Was thinking of ronal ls or turbos or maybe lenso bbs copies 7x15

Thanks for replies.

Been looking at this myself, I had spec'd out 205/45/15 with +20 offset. Seems like everything should fit according to all the calculators and stuff however if anybody has any real world info, like they already tried these and they defo don't fit would be good to know for sure. I'll be running JOM coilovers when I get her put back together. I would measure everything on the car myself except she's totally stripped to a shell right now and in the middle of getting restored.

Car had original 15"s on it running 195/60/15, 6jx15 ET35. And they fit no prob on full lock and no scuffing any arches because they sat back into the wheel well a fair bit, running stock suspension.