My company right now does OKRs with priority levels for each task. If you make it a P0 then you're saying it will get done this quarter baring really unexpected circumstances (ie: shit happens, we understand). P1 may not get done. P2 is even less likely to get done. This creates a natural distribution by holding the date constant and varying the work done by that date. Personally I find this more reliable and easier to communicate than holding the work constant and varying the date of each item.
To be honest, I also like that approach better, because in my experience the budget tends to be relatively fixed, but the work amount is often fairly flexible.
