Just a thought, it is possible to overfill the SP-445 slightly.

There is on my unit a small tab which equates to the top of the film (or maybe holder). This can be seen when looking down into the filler side.

I've developed quite a lot of film in my SP-445 and on the odd time I slightly overfilled I always felt the evenness wasn't quite right. Even though the films enlarged well. There was one time where I certainly overfilled and when inverting I had difficulty hearing and feeling the liquid moving around when inverting; mid developing I drained a small amount out and resumed developing.

Longer development times, mixed with quite good agitation make for good negatives; at least that is my experience. To that end I use D76 1:1 and my development time for Ilford FP4+ is between 12-14 minutes, dependent upon temperature.

Speaking of temperature, if you start at 24ºC with ambient at 18ºC then the drop is around 3.5ºC to 4ºC over 11 minutes. If using a print tray that holds some water at roughly 24ºC when you start developing and you return the Sp-445 to the tray in-between inversions, the temperature drop is about 1.5ºC. I usually measure around the ten minute mark.