Okay, everyone is mentioning doing something more complicated than tiers. The thing is that would complicate matters a lot matter. As doog mentioned before, the invested amount for each player is really easy to calculate, a recalculation needs to be done only when players invest or divest. The thing with a non-tier system is that there will be a hell lot more calculating that needs to be done. And taking the sum of all the invest amount times risk is really hard to do. It will need to be updated after each bet as each bet loses and gains differently.
That's why tiers is better as it helps simplifies calculations.
I have a compromise to offer. Maybe you could have 3 tiers (or more). And players can invest in a combination of tiers. 0.25%, 1%, 5%. Something like that. The highest one could be really high.
This is because you can invest in a combination of tiers. And the calculations wouldn't be extremely difficult as the server can bunch all the low risk accounts together, med, high etc.
I think this would actually be really dope, and very easy to implement. With that said, I'm not convinced that letting users input their own risk level, while more complicated to code, represents that much of an increase in computational difficulty for a computer. But I've mentioned a few times that I'm in favor of multiple investment buckets, if tiers are the difference between a single number denoting acceptable risk on the entire investment and being able to split your investment into higher and lower levels of risk without creating a new account, I would be very happy with that solution.