Try using tetenal regular Ultrafin (NOT ULTRAFIN +PLUS!!!).(posted 9407 days ago)I learnt this on a photgraphic workshop that was hosted by Fay Godwin who used this film/developer combination extensively!! Good enough for her ....goos enough for me.
I develop APX100 in a 1:30 dilution (not shown on Tetenal bottle) for 12 minutes. Presoak the films with water of the correct temp and agitate for 1 to 2 minutes constantly. Then pour pre soak water into a container holding the right amount of Ultrafin. The water will be discolored by the anti halation layer giving it a strange blue/grey tint. When it hits the concentrated ultrafin it will then go pink!! YES PINK!!! Shake briefly and then pour back into the film tank. Agitate for 30 seconds constantly then invert tank twice every 30 seconds from then on until the 12 minutes is done. You will be amazed at the quality and punch of the negatives!! I have used this film dev combination on APX100 in all formats...35mm, 120, and 5x4.
I use a simplified version of the zone system in all my exposures but I still develop my negs in this standard brew and always get very good printable results.