Thanks for looking Stephen. All is OK with the FB+F reading as I didn't make it sufficiently clear. The FB+F readings are stored as comments in the xls spreadsheet in cells G41 to K41. (those comments aren't visible on the screen capture, but in the actual xls file). They are 0.11, 0.12, 0.14, 0.15 and 0.14. all readings have a tolerance of +/- 0.01 so I am happy they are monotonically increasing after taking into account the tolerance.
The density values you read off were from cells G42 to K42 and are in fact those for the darkest step on the X-rite tablet (D=3.80) which I exposed alongside the 31 step Stouffer tablet. The value I entered into cell G42 is actually out of place and is understandably confusing (just ignore it if you can't understand what I've done after reading the rest of this). It results from a discontinuity of 2 stops as explained further on. The reason the film developed for 8 minutes has a higher measured density for all its steps including the one at cell G42 is because I was forced to read the entire set of values off a neg frame exposed 2 stops higher (1/15s rather than the nominal 1/60s I used for the remaining films) and then shift everything by two stops along the Y axis (vertical Transmission Density axis). This happened because one of the 5 films was ruined during the development as I tapped the tank to release the air bubbles the lid popped off and let stray light in to fog most of it ! I managed to recover from that scenario by cutting another roll in half and developing one half for 8 mins and the other half for some other time. I had filled all rolls by bracketing with 4 shots and the nominal exposure I chose to use in the spreadsheet (1/60s) ended up on one half of the cut roll not the other, so I had to use the 1/15s and compensate by shifting the y-axis values by 2 stops = 0.6 density units. I had to guess/interpolate some of the numbers because this method meant I was missing values for steps 26-31. Additionally the value of 0.16 in G41 was actually for step ((38.5-2x0.3)=32.5) not step 31 so the shifted value in G41 is slightly lower than what it really was, but for the sake of not upsetting the automated curve fitting equations Ralph set up I shaved a bit off it. It was the only point I compromised on from a data integrity point of view and could easily be explained by the fact that the ratio of my two shutter speeds (1/15)/(1/60) wasn't exactly 2.
Given the 8 min graph looks well placed compared to the others in the Family of Curves then I am happy my recovery steps were successful.