A lot has changed over the last decade or so. Yeast has come a long way both in the way makers recommend we handle it and the range available, especially in dry yeast.
A lot hasn't changed, and probably wont any time soon. The amount we need to pitch is still pretty much defined by the same equation, for ales its given as Number of cells = 0.4-1Million Cells/mL/oP. (1-1.65M/mL/oP for Lager)
That's a pretty wide range and I guess its arguable exactly where in the range you want to be. Low Oxygen (less reproduction) in the wort would push you toward the higher end as would brewing cooler as the yeast is going to work and reproduce slower. Lower populations means more esters in the finished beer (probably more Diacetyl to) that's why an ester driven beer like an Hefeweizen would be pitched at the lower end of the range (not underpitched as some home brewers say).
I suppose its fair to say the more stress the yeast is going to be under the more you should pitch at the start.
Brewing Big Beer (believe me 1.100 is big beer) is very much an at extremis when it comes to what we are asking the yeast to do, I would be pitching right up at the high end of the recommended range.
I would also be racking, there is practically no way known that the primary will be over in less than 2 weeks and its pretty well established that the oldest yeast will start to show signs off negative effects after about 14 days (primarily the release of A-Protease which will eventually degrade all the soluble proteins to peptides which will dramatically reduce the head holding potential of any beer - High Alcohol is fairly head negative to, so more head negative products is the last thing you want). Wait too much longer and you will start seeing signs of Yeast Autolysis and start getting off flavour's like burnt rubber and Vegemite.
Lets take a look at the wort and the yeast you are working with.
SafAle US-05 Says 50-80g/100L (HL).
From the above equation (you don't say how big your batch is so lets assume its 23L), an S.G. of 1.100 is ~25oP (oP being Plato the other way to measure SG), being a big beer lets pitch at 1Million Cells.
Number of Cells = 1EXP6*23,000*25 = 5.75EXP11
From the link above; Viable yeast > 1.0 *EXP10 cfu/g
5.75EXP11/1EXP10 = 57.5g, 5 packets (pretty clearly that 50-80g/HL applies to much weaker beer).
Next thing to have a look at is where this beer is going to stop, the Spec Sheet isn't very forthcoming with numbers, just general information.
Fermentis Tips and Tricks is a bit more useful, there are a couple of docs there that are well worth reading. Using US-05 about the highest apparent attenuation you are likely to get is 83% and that will require everything to go very well. From AA = Change in gravity/Original Gravity *100
83% = Change / 25 *100, 20.75oP or 83 Points. FG would be (100-83) 1.017 makes for a fairly intimidating 11% alcohol (ABV).
Personally I suspect an Extract brew is more likely to pull up in the mid to high 20's (1.025-1.029) just because if I were making an All Grain Barley Wine/RIS... I would be doing a lot of my mashing very cool (62-63oC) to make the wort as fermentable as I could. There wouldn't be any shortage of malt body left in a 1.100 beer, using LME that really is designed for making 1.048-1.050 beer wouldn't be my first choice.
I suspect you will finish up with a very sweet beer, It will taste better with age in this case probably several years.
I would rack at the end of the first week, get off any old under preforming yeast that is most likely to do harm later. To minimise O2 uptake and to transfer as clean as you can I would look at transferring from Tap to Tap, fit a hose between the two taps, make sure everything is really clean and spray it all (especially the tap on the first fermenter) with a 70% alcohol or strong peroxide sanitiser. Open both taps and let the working beer transfer slowly. there should be enough turbulence to create a pretty good CO2 blanket in the second fermenter as it fills, enough to minimise O2 pickup.
At present I am planning a couple of barley wine and RIS brews, just finished (this morning actually) rereading Barley Wine in the Classic Beer Styles Series. Haven't heard of
SafAle HA-18, at last a dry yeast made for really big beer. Shame its only available in 500g blocks, but still I just have to get me some of that! As we are fairly local, I might get a brick, happy to share some with a local and I know at least one other local who might be interested (yes I do mean you Coalminer).
Big beers are a challenge, require a lot of patience both in brewing and giving them time to mature, often years.
Bit like good Whiskey, what's a decade or two when it comes to great flavours.
Mark