According to me, in the HR department, you have to set slabs based on the grade or designation. Divide the entitlement into 4-5 segments (e.g., A - 800, B - 1200, C - 1500, D - 2000, E - Actual). Then, discuss with each department head and decide, based on their roles and responsibilities, which employee will fall into which segment.
In the policy, you have to clearly mention that any amount exceeding the bill amount will be borrowed by the employees themselves. State that these are the maximum limits. Additionally, be practical and allow for some exceptional cases, such as Sales employees, and mention that further approval will be needed from their HODs.
If you are asking specifically for a range, I must say it will solely depend on their roles and responsibilities. This information may be helpful for you.
Regards,
Gunjan