I agree with Liam.
We do it lilke this :
01 : Disk - spool (though in your case it'd be 30 day retention or 7day 4 cycle maybe)
02 : tape Weekly full - selective last full of the week (we retain these for a month)
03 : tape EOM - selective copy of most recent full backup. (we retain these infinitly)
You might st your disk retention a little longer if you can just sot he source data is available for the selectives when they go off. I had some issues where tapes had been removed and the disk had aged off so when the selective ran it was asking for tapes to be put back into our loaders.