If you have a 7"+ flat available. Also, the contact of a lap at f/3 asphere is different than a sphere, so some consideration is needed there...I go for smaller laps to handle that contact issue.
But yes...do DPAC if possible.
With the focal length being under 20" the mirror could be mounted in a OTA and mounted vertically and then one could use an oil flat and ultra bright LED as light source to do double pass testing. How I have figured these very fast mirrors is first get them too a clean sphere. then with the mirror on the bottom I place the center of the lap over the 70% of a do a W stroke taking the center the lap to about the 30% zone and out to the 90% zone. Using double I can monitor the amount of correction and if I need more in outer area for example I move the center of the W stroke outwards and go inward less.
The beauty of double pass is that it allows me to SEE exactly were the figure departs from parabola I want just like one does when using a standard Foucault test to see how the figure departs from a sphere, no measuring, just look. It also allows me to quickly determine if the stroke I'm using is making a positive difference or not.
Having taught mirror making classes for many years I have seen ATM's come to the classes with mirrors they have tried to figure for years and I do mean years using other test methods and with in a day finish their mirrors to show a nice null using double pass. The "magic" is that you can see were the figure departs vs inferring via other test methods that also have sources of errors that are not accounted for.