It took me a long time and research to figure out that when using flash, you are actually exposing for two things. You expose for the area in your background and you expose for the subject that is the main subject that you want lit by flash. You said that the room "will be a large hall, higher type ceiling, some natural light but not consistent". But you don't say what the subject is. Are you taking portraits? a group photo of people? a picture of the room? the furniture, artwork, etc.???

If taking portraits, check exposure settings for the room at a shutter speed within your cameras flash sync above 1/100th. (but below your max sync speed) Adjust the ISO until you get the depth of field you told us you'd like to keep your subjects at. In this case f/4 to f/5.6. If your ISO is 1600 so be it. I don't know why you said 1/100th is your shutter speed. You can use a slower shutter speed with a tripod and drag the shutter to expose the room and keep our ISO as low as 800.

If this is unacceptable then you may have to use more off camera lighting to properly light the room. In that case you can probably get your ISO as low as you like. Then you can take a flash reading with your meter (of your subject) and adjust to taste.

In a situation like this, you do want to let the camera and flash take care of things for you. The Nikon flash exposure system is excellent, so let it do its thing. If you have the camera on manual exposure mode (and manual ISO), but have the flash on TTL, then you control the ISO (noise), aperture (DoF), and shutter speed (movement blur), while the camera still gets the exposure right. When you look at the meter, it should be (-), so you would need more light, and the flash will supply that. How far (-) the meter is gives you an indication of how the flash and ambient light will balance. If the meter is at -1, then the two will be equal.

The D7000 can handle higher ISO than 800.
I shoot several events every year and have used the D7000. Fine camera. For what you are shooting, leave the flash on the camera.
"Off-Camera" means somewhere other than attached to the flash shoe and that's something you're not ready for now.

Below is a shot I did using the Nikon D7000 and an SB 800.
The camera was set on manual at ISO 1250, f/4, 1/20th sec. The flash was ttl...Nikon's system works great.

If I had cranked the shutter all the way to 1/250th, there would be no "ambience" to the photo...it would look like I had shot the photo in a cave, ie: a black background. You will have to strike a balance between what you comfortable hand holding (the flash will freeze the subject), any subject movement and how much you want to show in the background.

A flash modifier like a Lite-Scoop or a Lumiquest will help soften the light by making it bigger.
Those snap-on caps that come with some flashes will not soften the image on their own because they are still relatively small.

Re-read your post. If using TTL system will do all the work. No need to go 800 ISO although that will put less demand on the flash. Shoot SS 200 to kill ambient light and 4.5 or 5.6 will be great. 5.6-6.5 for broader focus range, DOF. Camera and flash will provide for correct exposure. Only variable will be to soften the light and create a larger light source. Solution is Rogue FlashBender or Gary Fong type diffuser or portable soft box , see lumiquest or light scoop.
My 1st choice for high ceiling would be Rogue or similar or soft box

I have a D7000 and SB-700 flash (SB-400 also). You don't have to adjust the exposure before you take the picture. The camera will first direct the flash to fire some pre-flashes. Based on the level of light that is reflected back, the camera will adjust the output level of the flash. It will then direct the flash to fire at that newly determined level while taking the picture. It does that even when the flash is being used off-camera through pulse modulation. If you use the flash off-camera you'll need to put your pop-up flash in commander mode in the menus or use the Nikon SU-800 commander unit. If the ceiling isn't too high you can also bounce flash off the ceiling to get a light that isn't harsh. I just got a great book on the subject. The book is: Nikon Creative Lighting System Digital Field Guide 3rd edition, Benjamin Edwards. Another great book is Bryan Peterson's Understanding Flash Photography.