Since we earn money each month, it is extremely tempting to increase the investment amount monthly. Probably this should be done until a minimal qualifying investment amount (maybe 50,000 USD or 150,000 USD) is reached. The benefits of flexible amounts are clear:
1. Ability to use new opportunities each month
2. Natural growth of the investment
3. Easily handled drawdown
Unfortunately this tactics makes it hard to monitor account profitability and tends to support risk-taking strategies by generating high momentary revenues and covering long-term draw-downs.
One may maintain relatively large cache position of ~25% rather than ~5% typically available on regular trading accounts to use opportunities and avoid risks, but this does not eliminate the basic revenue tracking complexity.
Fixed allocation on the other hand enables easy tracking of profitability and tends to boost more careful and long-term approach. They are more suited for accounts with tax benefits, where the amount cannot be easily modified.
It may be reasonable to have a third approach of keeping the main saving account outside investment account and investing new money upon large market corrections or spotting new opportunities. In this setup the money should probably be invested once a year, and maybe more risky years should be skipped.