This is how it works:
There are no rounded-in-F temps. Note: if you are using the Chart Neo app without Tempdrop, then always enter the raw F temp and not a rounded F temp.
The temp display can show Celsius numbers or Fahrenheit numbers, but it is the exact same rounded-in-C temp.
No, I recommended you continue to chart in your native temp scale. In Fahrenheit you use the number displayed as a range check, such as too high means possible fever and too low means too loose armband. This is most intuitively done in your native temp scale. So, the Fahrenheit number display is still useful even if you do not use the number to self-interpret your chart.
You do this by looking at the dot pattern or deviation pattern. The horizontal lines shown on the Chart Neo app chart are separated by 0.05ºC, which equals approximately 0.1ºF. This is 1 deviation or 1D. When you are looking for a temp to be 0.2ºC (approximately 0.4ºF) above the coverline, you are looking for a temp that is 4 deviations (4D) above the coverline. That means you count 4 lines above the coverline and look at that point or above to find it. This way of chart reading works for any chart displayed with a proper horizontal grid behind it, like most paper charts.
To read a BBT chart, you are first looking for the pattern of a temp that is at least 1D above the previous 6 temps. Remember, the coverline will be drawn Sensiplan-style, THROUGH the highest of previous lower 6 temps. Please note, if you use a charting method with a raised coverline, like TCOYF, then manually interpreting a chart displayed in the Chart Neo app may be more difficult, because you cannot change where the coverline is drawn.* However, if you use one of the many methods that marks the coverline through the highest of the pre-shift six temps, then self-interpreting a BBT chart from the Chart Neo app should be fairly straightforward once you learn to count the deviations. Next, after your first raised temp is recognized, to identify a full thermal shift pattern, you are looking for the second raised temp to again be at least 1D above the coverline (count 1 line up). Finally you want to see that third raised temp be at least 4D above the coverline (count 4 lines up).
In the Chart Neo app, you are able to self-read the BBT pattern and count where the graphed temp dots are in relation to one another. So, you are looking for things like at least a 4D change (counting up 4 lines from where the coverline is drawn). It is not necessary to subtract temp numbers to see if you get a change of 0.4F.
*More about the using TCOYF to read Chart Neo app BBT charts: If you still wish to evaluate your Chart Neo app BBT chart by TCOYF rules, then you will have to mentally add 1D (0.1ºF) to the TCOYF protocol, because the Sensiplan coverline is 1D lower than the TCOYF coverline. So, for example, when looking for a temp that is raised at least 0.3ºF above the coverline according to TCOYF rules, that turns into looking for a temp that is raised at least 0.4ºF (aka 4D) above the coverline in the Chart Neo app.