Drum Roller for 16 X 20

I am new to using a drum roller system. I have a 16x20 drum and have 2 questions.
1. Any idea how much chemical (oz) to use to print 1 print. Using developer for 1 shot don't want to waste chemical also want to get max use.
2. Also this may be the wrong place to ask this question, I'm obviously making 16x20 prints and my exposure time 4x5 neg f11 w/ 2 1/2 multigtarde filter are 90 sec.
Negs look pretty good, if anything little light. Does a multigrade filter make that much difference.

If it is a Cibachrome 16x20 tube/drum - 300 ml.

See this thread (at the end): (there was a url link here which no longer exists)

I use 100 ml for my 8x10 Cibachrome drums. I expect 60 ml might be enough, because 120 ml works well for my double 8x10 drum.

As for the exposure time, what enlarger head and light source are you using?