Calculate DNA amount - (Sep/30/2015 )

I’d like prepare a reaction mix (total volume: 120 uL) containing 250 ng genomic DNA. The DNA concentration of my sample is 51 ng/uL in 65 uL RNAse free water. Does it mean I have to add  4.9 uL from my sample to the reaction?

51 ng DNA -> 1 uL

250 ng DNA -> ? uL

250/51 = 4.9 uL

Is my calculation correct? 

Yes - a simple way to check is to ask how many 50's there are in 250 - there are 5... 51 is slightly more than 50 so there will be slightly fewer 51's in 250...