Forging - Some results from my experimentation (in General)

I assumed that the higher the NW of an item, the more it took to forge.
Some figures
The first two I did at an MPR of ~108k
The last at about ~111k

Leather Boots NW 12.5k
16.2 percent gained over a cycle of 13

Leather Cap NW 10k
11.04 percent gained over 12

So boots gain more than Cap per BA?

HXBow NW 45k
98.9 percent over 27

admittedly an extra 3k MPR, but it's nearly 4x the NW!!

So does each item have a 'forging PR'? and if so, do we know it?

I don't think you can go by percent.. but this is all just off the top of my head.

I think it depends on the items NW per +.

Vicious, some items have a "sweeter" cycle, meaning they fall on more even time cycles so that you need less BA per cycle gain. I had it down to a certain magic number for a bunch of items back on CB1, but I am not sure how much is the same here.

Basically, look at the NW gain per cycle (by multiplying the percentage progress by what it would cost the BS to go to next click). Compare that to your current BA cost (assuming no NUB or NCB), as that is a good indication of your current forging capability. Items with a good gain/BA ratio are favorable items to forge. Items with a lower ratio, not so much.

the higher the NW of an item, the more NW it takes for every consequent point to add...

Also every item has it's own optimal formula AND it's own base progress gain.

in other words, some items forge better than others.

In that case, is it acceptable to rent an item and do one cycle on it before returning, just to get a feel foe numbers?

It is accepted I think, you can't sell an item and forging 1 cycle on an item doesn't hurt the item itself so I can't see the harm in it :p