Best practice fillet this area also new file small object inch is mm!

Hi,
V5.

attached my object copy pasted from my new file small objects inches (and I DEFINITELY CHOSE THAT TEMPLATE, a default Rhino template, not one I made) into another small objects inches, but its minute. what the heck ???
When I look at the source properties>units it says mm yet I have been working in that, the small grid squares were 1 inch, I did wonder why entering text it said mm !!

I select fillet rad 0.4 and get such, all is good, why did a project I chose as small object inches go wierd ?

If it hadnt been for copying it out to another small objects inches I wouldnt have known.

How do I get out of that pickle ? Need the original file as units inches and all workings inches.

I bet all my dims will go massive or microscopic as well…

6.438 ( 6 7/16inch in fact) has become 0.253 after I paste to another small objects inches new file.

then the reason for this post, fillet to be rad 0.4inch, I explode and select two surfaces, but at the one end of it there is a curve, that area needs filling in, is there a way of doing this in the command and is exploding this the best way, is there not a fillet solid selecting solids ?

What is best way to lay in a fillet of 0.4inch rad there and cap off the ends. It was a casting from what I can see.
fillet 0.4inch method but file in mm.3dm (163.4 KB)

Steve