Based on what I use for an 8x10 (~22 drops ferric/22 drops pt/pd on Cot320) , the amount you used seems sufficient.

What light source are you using? 6 minutes does not seem long to me...my exposures are much longer with my LF negatives (more like 30 minutes) which tend to be very robust, but it is difficult to compare camera and digital negatives.

I will let those more experienced with digital negs answer. Good luck in your explorations!

Vaughn