The Rule of 72 which you've quoted here is an investment formula...

The Rule of 72 which you've quoted here is an investment formula that holds true - if you divide any number by 72 it will give you the length of time it takes for your money to double.

For instance:
72 / 10% = 7.2 years
72 / 9% = 8 years
72 / 8% = 9 years, etc

This is all fine and dandy, except that investment earnings are rarely, if ever, linear in nature. That is, you can't bank on getting 10% per annum on any investment year in, year out, so as to double your money in 7.2 years. This leads me to the real point that I want to make here.

You're assuming that we are in for a very long term period of high inflation, and based on that assumption, you've made a couple of personal investment decisions:

One, to buy heavily into gold bullion on the premise that the gold price will hold up well as an inflation hedge. For this to happen, you need the price of gold to go up higher than the rate of inflation. You've quoted that it takes the same number of gold ounces today to buy a house as it did 50 years ago, in 1972. That may be true for the last 50 years, as the price of gold was at a lowly US$80 an ounce in the early 70s before inflation took off, but it's definitely not true for the last ten years. You need about twice the gold ounces to buy a property today as you did in 2012, because the gold price today is roughly where it was then, at $1,800 an ounce, whilst house prices have pretty much doubled (or more) in that time.

Two, you're avoiding exposure to cash and fixed interest presumably because their very low single-digit returns will not hold up in an inflationary environment. You've decided to invest in high growth (heavily in local and international shares, property, infrastructure and private equity) at the same time as you're about to embark on drawing down your superannuation capital (via an allocated or account-based pension). Retirees need to be very wary of sequential risk, that is the likelihood that a large downturn in prices of growth assets hits right at the time they start drawing down their retirement capital. Say for example, you're 65 and have $1m in super, and you need an income of $50k p.a. (which is equal to the minimum legislated pension drawdown of 5% p.a). If you start drawing down income as the market peaks and then corrects heavily (as it did in late 2007 through to early 2009), you might find that in 12-18 months time, you've lost a big chunk of your retirement capital. If for example the high growth option delivers a minus 20% return in the next twelve months, (roughly $200k on $1m) and you also take out $50k income in that time, by this time next year, your account based pension balance will be about $750k. All of a sudden, if you still need an income of $50k in the second year of retirement, you're needing to draw down 6.7% of your account balance. This means it's going to be difficult for your capital to recover to $1m.

It certainly pays to measure twice and cut once, but you've got to know exactly what you're measuring and cutting.