Hi Vitali,
You have some good answers from others and I'll add my take on this. First I go for 20% cash like per Lichello in his later edition. I have a diversified portfolio with many AIM programs and I'm still able to add to my account so I find the 20% is plenty. I have one cash supply to feed all my programs, most of which are ETFs, and are unlikely not to recover over time. The 20% is in either money market fund which yields some 1.8% now and in high dividend ETFs that are not very volatile. In that way the "cash" does not act as too much of a drag on performance.
The Vealie is a good idea as it reacts to facts on the ground, but I've been using a different method of limiting cash buildup in bull markets in diversified ETFs. I increment the PC by 10-20% per year so my hold zone increases in time. This has worked really well in the current sustained bull market. I'm using the old AIM spreadsheet and it was easy to add another column with the % /year increment in PC.
The Vealie and/or incremental PC are also a good way of increasing the size of your AIM program during a rising market without adding cash to it.
A lot depends on the size of your portfolio, the number of ETFs you have, and whether you need to take cash our of your account or are adding to it. I always advise to stay away from individual stocks with AIM as it's too risky or at least keep those to a small fraction of total account.
Adam