How to plan budgets for your employees’ PC cost?

How do you do about planning your end-user IT hardware budget?

If it is a case of leasing program, there isn’t much to plan since it is simple leasing cost per year multiply by number of headcounts.

If it is a case of purchase program, I would find out the number of additional (nett new) headcount joining + the number of PC warranty expiring in the following year X the cost of your most expensive model of the PC (laptop or desktop).

Lastly, cost in 20% for hardware accessories spend like external harddisk requests, broken LCD monitors, memory sticks…etc

Together, this would form your end-user hardware IT budget.

Is there a better way to calculate this?