IIRC I read somewhere overburning on DVD is a feature best left alone. Ok on CD, but forget it on DVD. I once tried to add only 15MB or so to a DVD and it was a failure.
I dont know for sure, but the possibility you damage your writer is much higher than with CD AFAIK. Not sure though - but I think 4.38GB on a cd sized disc already pushed technology pretty to the limit, at home at least.
I think the reason you had to test how much you can overburn a CD-R was that on a CD-R, it is not possible to assign it a length longer than something I dont know. Maybe you already saw sth like a 90min CD-R showed up in an old burner as a 12 min CD or so. Hence, you had to overburn to find out how long you could write and enter it in the software manually. Not sure but is was sth like that.