OK, here is your solution:
Monday to Saturday, full backup. 6 jobs / tapes / sets. You can restore any day for a week and nothing beyond that.
If a user asks for a file from 2 weeks ago because they did not notice the file missing is out of luck. This will only happen on a file not touched for a while.
Suggestion (there is no right answer, it is up to you / budget):
Mon to Friday. Continue as you are.
Saturday Week 1
Saturday Week 2
Saturday Week 3
Saturday Week 4
So for the cost of 3 extra sets, you now have 4 weeks of backups.