The simple answer is you are using a 10% offset.
20v is what is available with no offsets.
At 100% offset, only 10v is available as the maximum amplitude for the generator is 10v in either direction. 100% of the waveform energy has been pushed above 0v. The baseline of the waveform is now at 5v.
To make this a bit easier to show I will now work with a 10v amplitude so we are not having to truncate our values being capped at +-10v
At 50% offset, the baseline of the waveform normally at 0v is now at 2.5v.
The waveform will go to +7.5v and down to -2.5v for a total of 10v.
At 25% offset, the baseline of the waveform will be at 1.25v
The waveform will go to +6.25v and down to -3.75v for a range of 10v.
What happens if we ask for 20v using a 50% offset?
The baseline is at 2.5v, but we can not go to +12.5v, we are capped at +10v. The negative swing could go down to -7.5v. However, we have to be symmetrical.
Since we have only +7.5v on our top end cycle, we now have to recalculate our bottom end. You would think at first that it would also go down 7.5v and so be -5v.
However, this is not the case. The 50% offset being capped by the voltage limitations recalculates the actual baseline taking into consideration how much amplitude we can afford to the waveform.
So instead of the baseline being 2.5v for 50%, it is shifted upwards.
You end up with a baseline of 3.335v, with 13.33v allocated to the waveform. That's a waveform with +6.665v and -6.665v from the baseline, giving you a top end of +10v and -3.33v on the bottom cycle.
So let's look at 10% offset. The baseline will be set at .5v initially.
At 10v amplitude, you will get +5.5v and -4.5v for a total of 10v.
At 20v amplitude, you would cap out at +10v, having only used 9.5v of the upswing. So the baseline is recalculated. You end up with a waveform with a baseline of 0.915v The software rounds this up to 0.92v, the generator rounded down to 0.91v
The top end caps at +10v, for a change of 9.09v in both direction giving you 18.18v from the software perspective. The generator rounding the baseline down to 0.91v has 9.08v in both directions giving 18.16v.
As for your other question, the calculator is designed to work with or without the 10k ohm resistor. You have to enter your current in mA which is best measured from your generator (see page 201, step 4). If you have the 10k ohm resistor in use, it will be accounted for in your measurement.
Please check the link below for more answers and thoughts: http://on.fb.me/1UuCZBT