I would simply work out what I could afford to pay each week/fortnight/month on bitcoin, $20, $50... $100 and once bought it goes straight into a paper wallet that I prmice myself I won't touch until retirement or a set target date say in 20+ years time.
The only stipulation is you have to keep buying no matter if the price goes up, down or sideways as regular as clock-work.
investment is saving in the long run. I think it will be more effective if we know the time for each purchase, so that later the profit gained will be greater. on the other hand, by investing means not disrupting our real work, and can run normally. so we have extra money to always invest. our long-term investment goal is to guarantee our old age financially free