David, your curve implies that the shoulder is as abrupt as the toe, which is generally not the case. It also requires that you catch the complete shoulder region with your measurement data points or you risk numerical instability in your curve fit.
For these reasons I strongly advise against this model and recommend you take another look at my square model.
Trying to be the best of whatever I am, even if what I am is no good.
CI Calculator Now Plots Dev/CI Chart and New Delta 100 DDX Charts
Following Bill's suggestion, I have added a Development Time/CI plot. It highlights, with horizontal lines, the CIs I would presently associate with different N-times. If your CI-N relationship is different, and if you use my R code, you can just pass those as a parameter, it is explained in the comments in the file.
As for real darkroom stuff, I have just run another test, at 4 min in DDX 1+4, to get the lowest contrast curve, which I was missing earlier. This is what my Delta 100 seems to be doing, I wonder how different it may be from others on the forum (I've only seen Michael 1974's plots):