What you are suggesting has proven to be a highly risky way to do it!
You would be surprised at the number of users that lost everything when one of the discs in the set went down.That is the price you pay for automatic disc spanning.
If these are important I would suggest that you manually divide the discs in to the size of the media you are going to use... Keep in mind that 8.5 GB DVD's are available as well as 25 and 50 GB BD discs. (suggest an M-Disc burner and meda )
I usually make up folders, Disc 1, Disc 2, etc. and start filling them until Media Capacity is reached, then start into the next one until you have all the files. Now you know how many discs you need and you are ready to go.
If you are prudent you will make another backup using something like Acronis along with a recovery disc. And one more on HDD.
Now you have 3 archives and as long as you don't keep them in the same place, it is pretty well foolproof