Digestion of Genomic DNA - (Apr/17/2012 )

Question is: You want to digest the genomic DNA to an average size of ~15kBP. You evaluate the usage of two restriction endonucleases, Sau3A (↓GATC is the cleavage position) and BamHI (G↓GATCC). The activity of both restriction endonucleases is measured in units (U), where 1 unit cleaves 1µg of bacteriophage lambda genomic DNA in 1 hour at 37 degrees centigrade.

How many units of Sau3A and BamHI do you require to digest 20µg genomic DNA to an average size of ~15kBP in 16 hour?

Additional Information: Bacteriophage lambda DNA contains 116 Sau3A and 5 BamHI cleavage sites.

------ If anyone can help answer or direct me to a website with any calculations like this it will be much appreciated? Cannot seem to find any information anywhere!

I mini prep my plasmids by Sambrook's conventional method and I get up to 10ug/ul some times. I fully digest my 10 ug/ul plasmid with 1 ul EcoRI from Fermentas within 3 hr. I can even digest this within 1 hr with their Fastdigest EcoRI enzyme.