I think it has been emphasized by many contributors here that there can never been an ideal percentage of funds that should be allocated to Bitcoin investment because it is something the individual should work out based on several factors. The percentage two individuals receiving the same salary will invest in Bitcoin differs because both might have different responsibilities. It is therefore not proper to allocate a percentage of personal income an investor will invest in Bitcoin. Another thing to also put in mind is that people have different priorities even as they plan to invest in Bitcoin. Someone might prioritize investing in Bitcoin and will allocate higher percentage than another one that might also have other priorities and can therefore go for lower percentage to be invested in Bitcoin.
What is most important is to invest an amount that will not represent so much financial burden on the investor so that the investment can be held for a long period of time because it is in long term investment that Bitcoin is best. This bring to mind the concept of DCA that allow the investor invest in a manner that will not be so much stress financially.
Bitcoin investment requires good personal planning and not based on the other person's idea of amount to invest because every individual has it own expenses and budget. Just invest according to your financial strength, the amount that you can afford to invest which is not going to stress you. When the understanding of the amount of money that should be invested in Bitcoin it makes every easy. It is not about the amount of money you invest in Bitcoin but what matters is if you can be consistent and be able to hodl that is what matters.
Since we have DCA method of investing their is no need for one trying to invest so much that will later be a burden to them, not enabling one to meet up with financial demands. Invest with amount that you can afford, this always ends with a successful outcome at the end.