Set Numerical value for Supply Allocation Rules instead of Percentage

Summary:

We have a requirement where we want to allocate supplies based on priority but High Priority Demand must be set to a number instead of percentage. High Priority Demand must be allocated a fixed quantity while the remaining supplies should be automatically be assigned to Low Priority Demand. We do not want to use percentages as this will always look at weekly supplies an re-distribute allocation based on the available/incoming supplies

How can this be achieved as there is no method available in GOP Manage Supply Allocation Rules to provide a Numeric value. Additionally, if this is possible, how will the system behave when the total supply falls below the number set in the threshold.