I think your maths /logic is mostly right, but you've laid out the problem slightly different than on page 162 (print edition), so to be clear:
portfolio value: £4M (based on 25% vol)
annual cash vol target: £1M
daily cash vol target = £62,500
Price vol: 1.33%
block value: $750
ICV: $997.50
fx: 0.67
IVV: £668.325
VOl scalar: 62500/668.325 = 93.52 contracts
subsystem position with a forecast of 20: 20 x 93.52 / 10 = 187 contracts
With an IDM of 2.5 this would be 187 * 2.5 = 468 contracts
Eithier (a) we're trading this crude oil with a bunch of other things, in which case we'd be multiplying by an instrument weight as well as the IDM; or (b) we're trading it by itself in which case the instrument weight is 100% and the IDM is 1.
For now let's assume it's (b) - a system with only crude oil. So the position is 187 contracts.
Now: Risk (at least in my world) shouldn't be confounded with margin
The daily risk of this position is 187 * 997.50 [IVV: expected daily risk in $ per contract) * 0.67 (fx) = £124,977 [the margin doesn't come into it]. This is almost exactly twice the daily cash vol target - by construction.
What about margin? Well the margin, as you say, is $4100*187 = $766,700 or £513,689. This is around 12.8% of the capital at risk (£4m).
Let's bring the IDM back in again. In other words I'm pretending we have loads of things trading, all exactly like crude with margins around 12.8% of capital with a forecast of 20. Now the peak margin usage will be around 2.5*12.8% = 32%. The average margin usage (forecast of 10) will be 16%. This is around half of what I use myself (around 30%, from page 18). Reason: (and this is the crux of your question) Crude oil is relatively margin friendly. Many other instruments require more margin than this.
Hope this makes more sense now.
GAT