Can someone confirm if I am doing the right thing.
If I want to produce a set size photo in Lightroom ie 10x8
1) set crop ratio to 4x5
Yes correct - set it to any ratio that is equivalent to 10x8. Or you can actually set it to 10x8 in custom.
Crop in LR is by Ratio - not size.
2) save as and resize to 10x8 at my required dpi ie 250 or 300
You cannot actually "Save As" in LR, you need to export.
But if you print with LR print module you set the resolution in the print output.
Then you do not have to save or export or resize.
Does lightroom resample up if the image crop is very tight in a similar way to Photoshop?
No and yes, LR does not alter the file.
I do understand that LR interpolates the out put to achieve the required DPI.
"Size" of the photo is not something that is really a concept in LR until you come to printing.
Hope this helps
See this video for more help