Hard to say really...alot of photography is trial and error. There are unlimited variables that can effect the outcome of the photo like the lens your using, the light, subject of the photograph...etc. Id probably say start with a low ISO like 100 with a f/9 aperture and go from there.
What are you shooting? How far is your subject? Keeping the ISO low is not always a good idea as it will demand more light from the flash. I would also not stop down too much because diffraction effects with the D70's APS sensor will cause softer pics.
I would leave it in P mode unless there is some thing specifically not working...90% of the time it works extremely well.
I have the SB 600 and there are a lot of considerations.
When you say increase DoF - by how much and at what distance are you shooting. Those will affect the bokeh (perceived DoF) and will also require at what power you need to set the flash at depending on how much you want things to light up.
Also for the flash, it depends on WHAT you want lit: background, no background, etc. This will dictate: TTL or iTTL.
I am still trying to learn as I don't use flash that much myself. It is very overwhelming the amount of permutations and combinations that are factored in with the flash.
Unfortunately there will be/is a lot of trial and error to learn and achieve the look you may want.
I assume you have a manual for the SB800. There should be examples in there. If not I strongly suggest you get the manual.
The little flash won't help you with landscapes though.
Start with measuring the ambient light only (test shoot without the flash) and decide how much of it you want in your photo.
You control the ambient/flash ratio knowing this simple fact: the shutter speed is irrelevant to the flash light component (unless you don't go higher than your camera synchro speed, usually 1/250s).
Do you want more ambient light? Lower the shutter speed.
Do you want more flash light? Increase the power on your flash.
Do you want more overall light (both flash and ambient)? Open the aperture or increase the Iso.
Just use your camera and your flash in manual mode (M), set Iso 400 as a starting point, and the aperture you prefer; shutter speed and flash power will be set accordingly.
It's a long story, David Hobby says it a lot better.