" ... so what is the best formula.."Wow Davy, big question!
There are so many things to consider to find the 'current' formula needed. Weather, light pollution, object, quality of optics/camera, etc.
For Lunar/Planetary: the best formula is a slow scope (preferably not Achromat) around f10 - f12 with a good quality Barlow or Powermate and 1/200th to 1/2000th second exposure rate. Small aperture is fine.
Small DSO's: medium to fast scope (short to medium Focal length giving f5 to f7.5) with no Focal reduction or barlow. Integration times from 2 seconds to 2 minutes. Small to medium aperture. If you have a good quality 0.5x Focal Reducer you can increase the aperture a bit more.
If you use a large aperture you may need a Barlow or Powermate.
Large DSO's: Fast to very fast scopes f4 to f7 with Focal reduction from 0.65x to 0.5x, very short focal length. If you want to use more aperture you may need to use two Focal reducers or fit Farstar to an SCT.
Integration times from 2 seconds to 3 minutes depending on object Surface brightness or magnitude.
The wider the FOV the brighter the objects will appear needing shorter integration times.
Richfeild and All-Sky: Camera lenses attached to video Astronomy cameras.
The main thing to remember when using Video is FAST FAST FAST.
The need is for short focal length.
One thing that must be remembered is that Video Astronomy is not reliant on Aperture like Astrophotography is.
As Astronomers we have always learnt that Aperture is King.
Well, with Video, the King takes a back seat
The Recipe is: The smaller the aperture, the the shorter the focal length, the faster the integration times.
Many people are producing fantastic results with the popular ED80 Apo Refractors and some are even using 60mm Apo's with success.
Also, many people produce fantastic results with SCT's and RC's with heavy Focal Reduction or Farstars fitted.
Small Aperture SCT's and good focal reduction seem to work better than large SCT's, unless that Large SCT has Farstar.
RC's with Focal Reduction are starting to gain popularity.
The quality of the camera becomes a major factor. A 1/3" CCTV security camera just cannot compete with a 1/2" dedicated Video Astronomy camera like the Mallincam Xtreme, but it will give reasonable to good results by patience and FAST good quality optics.
So, to answer your question Davy,
the best Formula depends on many factors.