Inches of down loft = what warmth ?

Try this from Western Mountaineering. Western Mountaineering

It mainly lists their bags and warmth ratings. These are variable, soo, depending on what you want in warmth, loft can change. WM is fairly consistent. Their bags are nearer the comfort rating of EU ratings.

As a rough estimate, you can use about 10F for each inch of loft starting at about 75F temps down to about 0F. It starts breaking down after that.
So, for a 25F bag you would need:
75F-25F=50F
At a per inch fill, you would need about 5″. This jives exactly with their chart.

For a 5F bag:
75F-5F=70F
At a per inch fill, you would need about 7″. This also jives.

However, for a -15 bag:
75F-(-15F)=90F
At a per inch fill, you would need about 9″. Yet they show -25 on their site.

Anyway, the estimate is close enough to use down to about 0F. Note that there are a LOT of variables that will effect the temp rating. Baffling, types, and how it is used. Draft tubes or lack of. Hoods, insulation, cinching. Foot box, pads used, wind resistance of the fabric, water resistance, ventilation, vapour barriers, etc. Not just 800 fill down.

With lower quality down, it is usually appropriate to add a bit. For every 20F, I would add another 1/2 inch for 600fill down. Because of the chopped feathers and stems included, This will lower the insulating value a bit. Never tried to figure out exactly what it was, though. Mostly, people worry about the weight.

Ray Jardine formula which is about the same as Richard Nisley's chart is

minimum temperature degrees F = 100 – 40 * loft in inches

where loft is the thickness of one layer of your sleeping bag

Typically, manufacturers quote loft as the thickness of top and bottom layers

For example, if the top of your sleeping bag is 2 inches thick it should be good down to 20 degrees F. three inches = -20 F

That's a rough estimate, different people are different, sleeping bag can be constructed better or worse,…

For down jackets does the same inch per warmth ratio apply?Or is there a different one?

You have the same approximately 4 clo per inch

Then you go to Richard's chart and find the appropriate line, say for "sitting and talking" you need 4 clo for 20 F, so a 1 inch loft down jacket would be good.

Or for strenuous backpacking (I can't read the text in the chart, something like that) you need 0.7 clo for 20 F. 0.7 clo is about what you get from a thin shirt and uninsulated jacket. So, for strenuous backpacking you don't need a down jacket.

I think 56-(loft in inches)*12.8 is a better predictor (where the loft if the thickness of the top layer only). I can't make the 100-40*loft function work for my sleeping bags.