It also comes down to your risk level, experience/skills in the...

It also comes down to your risk level, experience/skills in the business/topic, fundamental analysis effort (plus many other factors)...all to decide what percentage you should invest in any one company.

I think a set percentage across the board is pure rule setting for yourself around your risk management and that is perfectly good and sound advice especially late in life when most do need to play it safer.

If you have more life time on your side and more free dollars then you can mix these percentages up quite substantially to suit your own personal circumstances. All to work through with your financial advisor or your own head.