If you multiply 51x58=2958 sq inches
60x68=4080 sq inches
4080-2958=1122 sq inches
There are 1440 sq inches in a yard of fabric.
So it looks like you will need the equivalent of an additional 4 FQ, but keep in mind a portion, perhaps 1/4, of that will also be your background.
Me I would use 4 more FQ and purchase at least 1/2 yard extra background.
Remember you will need more border fabric too to accommodate the larger inside section.
This is a pretty pattern. I would go up one more row and across one more row, drawing it out on graph paper and decide from there is if it is big enough. I agree 51x58 is too small for a lap and too big for a baby. I like 72x72 for lap quilts.