How much DNA do you take forward in each step?

I am currently doing a ligation barcode library prep. I started with 600ng of each of my 21 different 5kb samples and after the end prep and bead purification, I have a range of masses, from 2.7ng to 500ng (measured by qubit). The protocol says to take an equimolar mass of each forward, but what would be the minimum cut-off you would use?