When travelling to eastern France from Dundee, we always take the PO Ferry form Hull to Zeebrugge. The times are more convenient, with departure approx 1900 and arrival at 0800 the next morning. No reclining seats on PO anymore, you must have a cabin. However, quite a large variety of cabins, with a basic 2 berth not overly costly (in the overall scheme of 'rip-off' ferry costs, that is!!). I would imagine you could get a crossing for near to the price of the Rosyth ferry. I have looked into using the Rosyth crossing previously, but have always found it to be very expensive when priced up. It takes approx 5 to 6 hours from Dundee down to Hull, that's going down the east side A1 and A19, to York and then across to Hull on the A1079, so would imagine it being similar for yourself.
Don't go the reclining seat option. Have used that on PO many moons ago and didn't really get any proper sleep prior to my 6 hour drive down through Belgium/Luxembourg/France. You feel grubby and knackered before you even start the drive! The ferries out of Hull are much larger than the Rosyth one and are well suited to a night crossing and a decent nights kip.
If you must go Rosyth, just pay the extra and get a cabin. You can't be short of a few bob if you drive a brand new GTI!! No offence intended!!