But if you are going to play around with different portfolio concepts - it sort of gets in the way. How do you backtest, project, pontificate, surmise and fill out the number on your spreadsheet if you don't have a "reason" for the 25% in cash.
I decided to cheat. I've declared that all of those cool things about cash will not be in my model.
Those great things about cash all boil down to 5 years of spending money - cash in the bank - separate and apart from any model.
Given that precondition, I put a minimum of 5% cash into any model for the collection of dividends and for general rebalancing lubrication.
Conveniently, if you assume Harry's portfolio returns about 4 percent over inflation - then you end up with about 25% in cash. Fancy that

For all of you back-testers and forecasters and economic/political worry warts out there - I just throw this out as another alternative to your model making. Don't forget the 5 years in the bank.