It does work with KiCad's transmission line calculator, and it's not as bad as I probably make it seem. I just don't like it, the way it is laid out irritates me. You will get a slightly different result than with other online calculators. Most online impedance calculators use simplified equations that make a few assumptions and therefore their results are only valid for a given trace width to dielectric height ratio. The one I posted earlier states that it is valid for a w/h ratio from 0.1 to 3.0, which is usually good enough.
To use the KiCad calculator for a differential pair I usually start off by estimating the Zo impedance and finding the trace width as a starting point. In your case we can start with an estimate of 50 ohms knowing that the Zodd impedance will be reduced by both coupling and solder mask. Using the "Microstrip Line" calculator enter your substrate details, namely the dielectric constant (Er)(4.2), the dielectric height (H) (0.18 mm) and the trace thickness (T)(0.7 mils). You can ignore the rest of the substrate parameters. I used 4.2 for the dielectric constant, you will need to check with your fab for the prepreg that will be used for your board. Now enter the target impedance for Zo (50) and click "Synthesize". It calculates the trace width as 13.5 mil which we can round to 14 mil. After changing W to 14 mil click "Analyze" and you can see Zo is now calculated to be 49 ohms. This is good enough so we can now switch to the "Coupled Microstrip Line" calculator. Enter the same substrate parameters and 14 mil for W. For tight coupling we can start with S = W so enter 14 mil for S as well. Now if you click "Analyze" it calculates Zodd to be 46.5 ohms. Since the solder mask will reduce this by 1 to 2 ohms to about 45 ohms giving a Zdiff of 90 ohms, we're done.
I should mention that I made a mistake in an earlier post. When I told you that your Zodd target of 45 was unrealistic, I was obviously mistaken. For some reason I was thinking you were setting a target of 45 for Zo which is a common mistake. But after reading the post again I realize that you were indeed correct, sorry about that.
Here's a quick explanation just in case I have managed to confuse you, or anyone else reading this.
Zo is the impedance of a single trace on it's own without any coupling.
Zodd is the impedance of a single trace of a coupled pair when driven differentially, that is equal amplitude but opposite polarity.
Zeven is the impedance of a single trace of a coupled pair when driven in common mode, that is equal amplitude of the same polarity.
Zdiff is the impedance between the two traces of a differential pair which equals 2 x Zodd
Zcom is the impedance between the two traces of a common mode pair which equals Zeven / 2
Edit: You may have noticed that the mantaro calculators give a slightly different result of 12 mil and 12 mil.