I do love these threads!
I'm not going to check the math either, I'll just ramble on as I usually do.
In my most humble of opinions, there is only one reliable way to determine your minimum fueling requirements. That is to provide enough fuel to cover the maximum mass flow rate your installed (or future planned) turbo supports. I do not recomend sizing fuel system components based on some given boost pressure, ET, horsepower number, etc.
The reason is that (I feel) you want enough fuel capacity available to cover you in the worst case scenario, and that is maximum boost, wether set on the boost controller by the owner, or achieved accidentally because of a malfuction (like the MBC line melting on the exhaust manifold, etc). I'm willing to bet that in the vast majority of cases where an engine blew due to excessive boost (malfunction), lack of fuel was the cause. If you have the fuel available, and things go wrong, the ECU will see the extra air and add the fuel, giving you a much better chance of getting by with your shit intact. Another obvious advantage to doing things this way is that once you grow some balls, dump in the race gas, and crank things up to max out your current setup, you know you have enough fuel to do it. Being limited by a (expensive) turbo is one thing, but being held back by a set of 300 dollar injectors when you spent 295 for the next size down is just stoopid.
The 950 recomendation is for people with DSMlink or other tuning solutions that allow for large injector size compensation. If 950s run like stock, why go smaller? They may be "overkill," but there is no real disadvantage. Conversely, there are plenty of disadvantages to going too small.
All that being said, size your fuel system based on the maximum mass flow of the turbo (lb/min or equivelent, not CFM), the Specific Gravity of the fuel being used (typical pump gas is .75, race fuels .70 to .73), base fuel pressure, and air:fuel ratio. I'm in no real mood to go into the mathematics, but myself or others may have done so here in the past. To put it basically, find your mass fuel flow requirement based on your air fuel ratio, use the density of the fuel to determine how much volume flow you need to get that mass flow, and with a bunch of unit conversions arrive at some value in cc/min. It's a lot simpler than it sounds luckily. [Edit> Note that these calculations use REAL numbers as measured on people's cars, not some obscure BSFC estimates, huge fudge factors, etc. I have found that doing the math in this way results in accuracy to within .1 AFR, less than 1% IDC, etc.]
To use your 20g situation as an example, pump gas, target AFR of 11:1, and 42 psi base fuel pressure, 650s would see 101.7% IDCs. With race gas (.73) and 12:1 AFR, 95% IDC. This is obviously the very lowest you could get away with.
To provide some mass flow ratings for other turbos (as measured on my car in most cases):
14b 30 lbs
small 16g 36 lbs
big 16g 39 lbs
evo 16g 42 lbs
20g 48 lbs
EVO Green 49 lbs
50 trim/Green 50 lbs
GT3076R/3052 52 lbs
60-1/Red 62 lbs
35R/GT14 65 lbs
T67/GT37R/3575 75 lbs
Using typical values for AFR, fuel, etc:
450s - 32 lbs/min
550s - 40 lbs/min
650s - 47 lbs/min
750s - 55 lbs/min
850s - 62 lbs/min
950s - 69 lbs/min
1000 - 72 lbs/min
1600 - 116 lbs/min
You can see another reason the 950s are so popular, they support the 65 lb wheel in the GT35R and other turbos it's used in (GT14, 3065, 3565, etc), which is the largest 99% of DSMers go, even on strokers. It's also worth mentioning that x80 injectors flow like x50 injectors despite the name. Last I looked into it I recall them being rated at 45 psi or higher, which gives them the higher flow rating. The x80 injectors invariably act just like x50s, so choose accordingly.
And for the sake of being thorough, the same math applies to fuel pumps, but it's more difficult since boost and therefore fuel pressure affects the flow. But as a general rule, for those not inclined to figure it out right down to the exact pressure, at 30 psi (common stopping point for most poeple), the intank pumps support:
1g - 17 lbs/min
2g - 27
190- 31
EVO8- 34
255LP- 54
255HP- 64
Supra- 73
Etc...
Hopefully something in this bloody mess is useful to someone...