Old Gregq said: In fact, I found it highly advisable to stay the hell away from 70ml because it's right there on the margins: start pushing film too much, or not developing frequently enough, and developing activitiy drops below acceptable threshold (becomes impossible to use with films like Delta 3200 or Fomapan 400).
I had a similar opinion. I cannot say the same opinion as you, because I use the Foma Excel in replenisment mode. Therefore, take my result with a reservation. I also increased the volume up to 100 ml, and this was because I observed a gradual decrease in activity. At that level, I poured out the working solution several times and replaced it with a new one. A few such attempts always ended in failure, and after about 20 rolls (edit: in a full seasoned solution, i.e. about 30 rolls in a total), I felt that replenishing didn't work. I could have blame anything as a reason - an un-original developer first, an inappropriate KODAK recommendation, and in fact, nothing showed that it could be my own fault.
Then, about a year ago, I read here on Photrio a discussion that I think was started by Bornmental (are you still here?). Someone in the thread described how carefully he proceeds with the replenisher measuring and filling. That's where I realized my mistake. I proceeded by adding exactly 70 ml of replenisher to the working solution bottle and then refilling the bottle by the rest of currently used solution from tank to the top. However, there was a spill of excess working solution outside the bottle. But in doing so, I lost control of the concentration of the new replenisher in the bottle. I started a new working solution and consistently carried out refilling and measuring in a measuring cylinder outside the bottle. Since then, even with Foma Excel, the working solution has worked steadily in replenisment mode at 70 ml as expected. My solution is now more than six months old and more than 50 films have passed through it.