I found an online calculator that is useful called "Calculate uncertainty of ReturnLoss and VSWR given coupler directivity"
https://owenduffy.net/calc/rlu.htmIt gives answers consistent with what you would derive with more work using the chart from the Electronic Design article linked earlier here -
https://www.electronicdesign.com/home/article/21201015/directional-power-measurements-and-the-effects-of-directivityFor example, say you have a forward power of 100W and reverse of 4W. That calculates to be a SWR of 1.50 on paper with no other considerations.
But you want to get the errors for an individual coupler directivity so you can know what the SWR error might be. If you look at the chart of reflected power, forward power, and VSWR you will see the following (for a VSWR of 1.5 and directivity of 25 dB):
Forward Power error band: ~ -3% to +3%
Reverse Power error band: ~ -50% to +62%
So for 100W forward the error band yields: 97W to 103W
For 4W reverse the error band yields: 2W to 6.5W
Using the "Calculate uncertainty of Return Loss and VSWR given coupler directivity" calculator let's plug in some numbers using a directivity of 25 dB and VSWR of 1.5:
The calculated error band goes from a VSWR of 1.34 up to 1.69.
Now lets calculate VSWR from the errors derived from the chart.
Using the max positive errors of 103W F and 6.5W Rev we get a VSWR of 1.67
Max negative errors 97W F and 2W rev we get a VSWR of 1.34
a 3rd combination 103W F 2W R: 1.32
finally the last combination 97W F 6.5W R: 1.69
The VSWR errors the calculator gives us matches very closely what you can derive with more trouble manually in several steps from the Electronics Design chart (which is a little hard to read exact values from, hence a small error).
So for me that calculator is a good way to know what the VSWR errors will be for the coupler I'm using, and so can be included in the specifications of the "product". By the way this type of specification (VSWR errors per band) is nothing I've even seen on any piece of ham gear I've ever run across.
But hams really don't care what the exact SWR reading is anyway - they're not trying for SWR 1.45 or some other exact value. They just want it to go down as low as it can and know it's not what they consider "high" such as 2 or 3 (ish).