Resizing using Lightroom (1 Viewer)

Firstly apologies for probably a simple question but I'm trying out different software and still learning stuff so here goes.
I like to resize my photos for posting on websites and Flickr etc and found this much easier to do using Elements or Photoshop than Lightroom which I'm trying out at the moment.
Am I right to assume this is best done by using File and export and using the Image sizing boxes.
At the moment I am trying to resize some photos to approx 620kb for my Flickr page and have the width and height box ticked.
I set resolution to 300ppi and basically alter Width and Height trying to achieve a file of approx 620kb.
This is however very hit and miss and takes me ages. One minute I'm at 900kb and then a tiny adjustment I'm down to say 300kb.
Any advice on how to consistently produce the best quality image (for Flickr) with a file size of approx 620kb using Lightroom appreciated.
I also want to produce best quality Jpegs for printing with a file size of maybe 6-8Mb but this will presumably use the same principles as above.
On a separate issue is it possible to move an object in Lightroom in a similar way to Photoshop ?
In PS I would select using the Lasso tool and move it to the centre or elsewhere where I wanted. I cannot see how to do this in Lightroom.

Your help and advice is appreciated - Thanks

Yes, Export is the right tool in Lightroom to create copies of your photos for publishing on Flickr.

There are 2 factors that effect jpeg image size - the dimensions in pixels and the quality (how much it is compressed).

To get to 650k, first decide what dimensions you want the image - for example, 3000 pixels on the longest edge. Then you tell Lightroom to target that file size.

In the export dialog, set the following:

File Settings
Image Format = JPEG
Color Space = sRGB
Tick 'Limit File Size To' and set it to 620K

Image Sizing
Tick 'Resize to Fit' and 'Don't Enlarge'. Select 'Long Edge'
Set the size to 3000 pixels and leave resolution as is (it is not relevant to online images).

That should get you a file about 620k. If you increase the dimensions of the export to 4000 pixels (assuming the original is bigger than that), Lightroom will automatically reduce the quality of the JPEG to keep the file within 620k. If you reduce the pixel dimension, Lightroom will increase the quality but you will have a 'smaller' photo.

Lightroom does not support 'pixel moving' edits as you describe. This is something you have to do in Photoshop.

Hope that helps