Need to fire-form cases for my 38-40 Remington. Have cases cut down from from both 30-40 Govt and 303 Brit. Both cut & trimmed to 1.79". For whatever reason, the 303 Brit cases chamber more easily than do the 30-40 Gov't. Whatever.

Anyway, my plan is to anneal the cases, then load up 10gr of whatever useless smokeless powder I have on the shelf and the rest of the case full of oatmeal or whatever useless dried cereal I also have on the shelf, put a wad in the end and make it go 'bang'. Does this seem like a reasonable/workable plan?

Thanks. How about 15-16gr of 5744?. 
45-70 load data for 5744 is in the 20-25gr range.

I just use my standard smokeless load and fill the rest of the case with cream of wheat and place a vegetable fiber wad on top of it.

5744 will need more resistance than any filler can provide.  Like a bullet.  Otherwise it will just go pfffft!

Glenn, the Krag brass is most likely slightly larger right in front of the rim. That may be the difference you are seeing. 
Pistol powders work best for fire forming,  8-10 grains of Unique generally does the trick with the case full of whichever filler you choose. It may take two firings to fully form them. The 2nd firing should be with a case full of black and a bullet. Best of luck to you.

There doesn't seem to be much to fire form. If there are no neck or throat or other chambering issues with the assembled round, I'd just load and start enjoying the shooting. Could always load down just a little on the first firing. I think fire forming with fillers and no bullet has a spot use for case forming challenges, but for a low pressure, low velocity cast bullet BP type round, it's lost time and components when there could be familiarizing with the rifle before developing loads if even needed?

Remembering my first K-Hornet.   I just fired regular factory Hornet ammo through it to get the brass, (in a T/C Gen I Contender).  It even shot decent groups.  I've read that this  happens with most if not all of the Ackley Improved cartridges.   

The upside of using fast powder and Cream of Wheat being that you don't have to go the range, of course.

I used to fire form quite a bit years ago when everything I was into was wildcat especially Ackley. Nothing could be easier than straight wall or light taper shouldered brass like the Hornet 32-20 etc. But you need fast powder and either a light bullet or plug to generate enough pressure to iron the case. A very very light lube of the chamber walls will encourage brass flow but I mean a light wipe only 

With shouldered and especially sharp shouldered rifle cases you absolutely need an almost a full power load with a snugly seated bullet to form the shoulder properly for head spacing. Again very light lube and nothing on the case especially near the shoulder. From my experience most fire forming problems come from not enough charge or pressure. Remember a good portion of the chamber pressure is being reassigned to forming the brass and never gets into the barrel. 

I did some testing years ago where I was working with the Ackley 30-06 Improved. Hot loads in factory brass 165 gr. bullets that chrono’d 2950 +- with H-450. Primers started to flatten so I could see the edge. Same load, same bullet and brass shot out of a new Ackley Imp chamber. Chrono reads approx 2450, primers perfect and brass nicely formed crisp shoulders with a bit of carbon smoke on the necks. 

Rick